[Pw_forum] vc-relax time

Discussion:

Masoud Nahali

2010-07-12 18:24:30 UTC

Dear Quantum Espresso Users

I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It takes 20
minutes with parallel running by 4 CPUs. Then I used the exact optimized
cell parameters (obtained from vc-relaxed calculation) to make a (2*2) slab
of graphite with 3 layers and I expected to see the results in a few
minutes. But amazingly it took 17 hours to complete. 48 steps were done in
the calculation for vc-relaxing the cell which have the parameters that had
been optimized before. The cell parameters only change a very bit in the
current vc-relaxing the (2*2) slab. I appreciate if one explain the physical
procedure of vc-relaxing and the reason of the time needed for the
computation.

input file:

CONTROL
calculation = "vc-relax",
pseudo_dir = "/home/koa/soft/qe4.2/
espresso-4.2/pseudo",
outdir = "/home/koa/tmp",
etot_conv_thr= 1.0D-4,
forc_conv_thr= 1.0D-3,
dt=80,
/
&SYSTEM
ibrav = 4,
a = 2.4579,
b = 2.4579,
c = 16.3069,
cosab = -0.5,
cosac = 1.0,
cosbc = 1.0,
nat = 6,
ntyp = 1,
ecutwfc = 40.D0,
ecutrho = 480.D0,
occupations = 'smearing'
smearing ='mp',
degauss = 0.03,
nspin = 2,
starting_magnetization(1)= 0.003,
london=.true.,
/
&ELECTRONS
conv_thr = 1.D-6,
mixing_beta = 0.7D0,
diagonalization = "david",
/
&IONS
ion_dynamics="cg"
/
&CELL
cell_dynamics = 'damp-w',
press = 0.0,
/
ATOMIC_SPECIES
C 12.0107 C.pbe-rrkjus.UPF
ATOMIC_POSITIONS {angstrom}
C 0.00000000 0.00000000 0.00000000 1 1 0
C 0.00000000 1.41908472 0.00000000
C 0.00000000 0.00000000 3.15347111
C 11.22896342 0.70954236 3.15347111
C 0.00000000 0.00000000 6.30694222
C 0.00000000 1.41908472 6.30694222
K_POINTS {automatic}
4 4 1 1 1 1

Sincerely Yours
Masoud Nahali
SUT
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Gabriele Sclauzero

2010-07-13 07:52:15 UTC

Permalink

Dear Masoud,

first I would suggest you to use bfgs as the algorithm for both ions and cell dynamics. Excepted particular cases, it should reach the minimum much faster.

Also, why do you specify the cell with such an unusual way. You simply need celldm(1) and celldm(3) with ibrav=4 if you want to describe an hexagonal lattice. Other suggestions: your ecutrho looks really large to me, do you really need it. On the other hand, degauss might be too large to describe a spin-polarized system.

Then, are you sure that you have built correctly your supercell? It looks like there are some C-C bonds much shorter that others in the central graphene plane (1.2 instead of 1.4 angs). Please check again your structure.
In general, you can expect that if you relax the atoms in the supercell some kind of surface-reconstruction may appear, since you leave more freedom to atoms to rearrange in structures with larger periodicity. I don't think this is the case for graphite, but you may find some ripples (as you mentioned in your earlier emails, if I am not wrong) if the C-C bonds are at a distance shorter than the theoretical equilibrium distance (I guess).

HTH

GS

Post by Masoud Nahali
Dear Quantum Espresso Users
I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It takes 20 minutes with parallel running by 4 CPUs. Then I used the exact optimized cell parameters (obtained from vc-relaxed calculation) to make a (2*2) slab of graphite with 3 layers and I expected to see the results in a few minutes. But amazingly it took 17 hours to complete. 48 steps were done in the calculation for vc-relaxing the cell which have the parameters that had been optimized before. The cell parameters only change a very bit in the current vc-relaxing the (2*2) slab. I appreciate if one explain the physical procedure of vc-relaxing and the reason of the time needed for the computation.
CONTROL
calculation = "vc-relax",
pseudo_dir = "/home/koa/soft/qe4.2/
espresso-4.2/pseudo",
outdir = "/home/koa/tmp",
etot_conv_thr= 1.0D-4,
forc_conv_thr= 1.0D-3,
dt=80,
/
&SYSTEM
ibrav = 4,
a = 2.4579,
b = 2.4579,
c = 16.3069,
cosab = -0.5,
cosac = 1.0,
cosbc = 1.0,
nat = 6,
ntyp = 1,
ecutwfc = 40.D0,
ecutrho = 480.D0,
occupations = 'smearing'
smearing ='mp',
degauss = 0.03,
nspin = 2,
starting_magnetization(1)= 0.003,
london=.true.,
/
&ELECTRONS
conv_thr = 1.D-6,
mixing_beta = 0.7D0,
diagonalization = "david",
/
&IONS
ion_dynamics="cg"
/
&CELL
cell_dynamics = 'damp-w',
press = 0.0,
/
ATOMIC_SPECIES
C 12.0107 C.pbe-rrkjus.UPF
ATOMIC_POSITIONS {angstrom}
C 0.00000000 0.00000000 0.00000000 1 1 0
C 0.00000000 1.41908472 0.00000000
C 0.00000000 0.00000000 3.15347111
C 11.22896342 0.70954236 3.15347111
C 0.00000000 0.00000000 6.30694222
C 0.00000000 1.41908472 6.30694222
K_POINTS {automatic}
4 4 1 1 1 1
Sincerely Yours
Masoud Nahali
SUT
_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://www.democritos.it/mailman/listinfo/pw_forum

? Gabriele Sclauzero, EPFL SB ITP CSEA
PH H2 462, Station 3, CH-1015 Lausanne

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Masoud Nahali

2010-07-13 10:49:53 UTC

Permalink

Dear Gabriele Sclauzero and pwscf Users

Many thanks for your attentions. The cell dimension and the positions of
the atoms are exactly correct. Yes, indeed the high ecutrho is important for
ultrasoft pseudopotentials. About the rippling : It was my mistake in
selecting a wrong pseudopotential which has a hole. Nicola had explained it
before and accordingly I solved it. The graphite surface is not ripple. When
I use the "max_second=6000 and dt=150" the job completes very fast as the
example of pwscf. Is the using of such keyboards plausible?
As I mentioned before I had used the optimized cell parameters of (1*1
slab) for vc-relaxing the (2*2 slab) and I expected to see the results very
soon but the calculation was time consuming while there was only a very very
bit change of the cell dimensions during this 16 hours. There is only 1-3
iterations per each step in the output file and each of them was time
consuming. The job was completed after about 50 steps.

-------------------------------------------------------------------------

Post by Gabriele Sclauzero
Dear Masoud,
first I would suggest you to use bfgs as the algorithm for both ions and
cell dynamics. Excepted particular cases, it should reach the minimum much
faster.
Also, why do you specify the cell with such an unusual way. You simply need
celldm(1) and celldm(3) with ibrav=4 if you want to describe an hexagonal
lattice. Other suggestions: your ecutrho looks really large to me, do you
really need it. On the other hand, degauss might be too large to describe a
spin-polarized system.
Then, are you sure that you have built correctly your supercell? It looks
like there are some C-C bonds much shorter that others in the central
graphene plane (1.2 instead of 1.4 angs). Please check again your structure.
In general, you can expect that if you relax the atoms in the supercell
some kind of surface-reconstruction may appear, since you leave more freedom
to atoms to rearrange in structures with larger periodicity. I don't think
this is the case for graphite, but you may find some ripples (as you
mentioned in your earlier emails, if I am not wrong) if the C-C bonds are at
a distance shorter than the theoretical equilibrium distance (I guess).
HTH

------------------------------------------------------------------------------------------------
Dear Quantum Espresso Users

Post by Gabriele Sclauzero
I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It takes 20

minutes with parallel running by 4 CPUs. Then I used the exact optimized
cell parameters (obtained from vc-relaxed calculation) to make a (2*2) slab
of graphite with 3 layers and I expected to see the results in a few
minutes. But amazingly it took 17 hours to complete. 48 steps were done in
the calculation for vc-relaxing the cell which have the parameters that had
been optimized before. The cell parameters only change a very bit in the
current vc-relaxing the (2*2) slab. I appreciate if one explain the physical
procedure of vc-relaxing and the reason of the time needed for the
computation.

Post by Gabriele Sclauzero
CONTROL
calculation = "vc-relax",
pseudo_dir = "/home/koa/soft/qe4.2/
espresso-4.2/pseudo",
outdir = "/home/koa/tmp",
etot_conv_thr= 1.0D-4,
forc_conv_thr= 1.0D-3,
dt=80,
/
&SYSTEM
ibrav = 4,
a = 2.4579,
b = 2.4579,
c = 16.3069,
cosab = -0.5,
cosac = 1.0,
cosbc = 1.0,
nat = 6,
ntyp = 1,
ecutwfc = 40.D0,
ecutrho = 480.D0,
occupations = 'smearing'
smearing ='mp',
degauss = 0.03,
nspin = 2,
starting_magnetization(1)= 0.003,
london=.true.,
/
&ELECTRONS
conv_thr = 1.D-6,
mixing_beta = 0.7D0,
diagonalization = "david",
/
&IONS
ion_dynamics="cg"
/
&CELL
cell_dynamics = 'damp-w',
press = 0.0,
/
ATOMIC_SPECIES
C 12.0107 C.pbe-rrkjus.UPF
ATOMIC_POSITIONS {angstrom}
C 0.00000000 0.00000000 0.00000000 1 1 0
C 0.00000000 1.41908472 0.00000000
C 0.00000000 0.00000000 3.15347111
C 11.22896342 0.70954236 3.15347111
C 0.00000000 0.00000000 6.30694222
C 0.00000000 1.41908472 6.30694222
K_POINTS {automatic}
4 4 1 1 1 1

Sincerely Yours
Masoud Nahali
SUT
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Gabriele Sclauzero

2010-07-13 11:02:15 UTC

Permalink

Post by Masoud Nahali
Dear Gabriele Sclauzero and pwscf Users
Many thanks for your attentions. The cell dimension and
the positions of the atoms are exactly correct.

Are you completely sure? You can find as an attachment a picture (from
xcrysden) of the central graphene layer according to the positions given
in your input. You can now judge if it looks like a honeycomb lattice.

Post by Masoud Nahali
Yes, indeed the high ecutrho is important for ultrasoft
pseudopotentials. About the rippling : It was my mistake in selecting
a wrong pseudopotential which has a hole. Nicola had explained it
before and accordingly I solved it. The graphite surface is not
ripple. When I use the "max_second=6000 and dt=150" the job completes
very fast as the example of pwscf. Is the using of such keyboards
plausible?

Please spend some time reading Doc/INPUT_PW to understand the meaning of
the _keywords_. dt is used only for molecular dynamics runs, not
relaxations, while max_seconds has nothing to do with how fast your job
is completed.

GS

Post by Masoud Nahali
As I mentioned before I had used the optimized cell parameters
of (1*1 slab) for vc-relaxing the (2*2 slab) and I expected to see the
results very soon but the calculation was time consuming while there
was only a very very bit change of the cell dimensions during this 16
hours. There is only 1-3 iterations per each step in the output file
and each of them was time consuming. The job was completed after about
50 steps.
-------------------------------------------------------------------------
Dear Masoud,
first I would suggest you to use bfgs as the algorithm for both
ions and cell dynamics. Excepted particular cases, it should reach
the minimum much faster.
Also, why do you specify the cell with such an unusual way. You
simply need celldm(1) and celldm(3) with ibrav=4 if you want to
describe an hexagonal lattice. Other suggestions: your ecutrho
looks really large to me, do you really need it. On the other
hand, degauss might be too large to describe a spin-polarized system.
Then, are you sure that you have built correctly your supercell?
It looks like there are some C-C bonds much shorter that others in
the central graphene plane (1.2 instead of 1.4 angs). Please check
again your structure.
In general, you can expect that if you relax the atoms in the
supercell some kind of surface-reconstruction may appear, since
you leave more freedom to atoms to rearrange in structures with
larger periodicity. I don't think this is the case for graphite,
but you may find some ripples (as you mentioned in your earlier
emails, if I am not wrong) if the C-C bonds are at a distance
shorter than the theoretical equilibrium distance (I guess).
HTH
------------------------------------------------------------------------------------------------
Dear Quantum Espresso Users

Post by Masoud Nahali
I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It

takes 20 minutes with parallel running by 4 CPUs. Then I used the
exact optimized cell parameters (obtained from vc-relaxed calculation)
to make a (2*2) slab of graphite with 3 layers and I expected to see
the results in a few minutes. But amazingly it took 17 hours to
complete. 48 steps were done in the calculation for vc-relaxing the
cell which have the parameters that had been optimized before. The
cell parameters only change a very bit in the current vc-relaxing the
(2*2) slab. I appreciate if one explain the physical procedure of
vc-relaxing and the reason of the time needed for the computation.

Post by Masoud Nahali
CONTROL
calculation = "vc-relax",
pseudo_dir = "/home/koa/soft/qe4.2/
espresso-4.2/pseudo",
outdir = "/home/koa/tmp",
etot_conv_thr= 1.0D-4,
forc_conv_thr= 1.0D-3,
dt=80,
/
&SYSTEM
ibrav = 4,
a = 2.4579,
b = 2.4579,
c = 16.3069,
cosab = -0.5,
cosac = 1.0,
cosbc = 1.0,
nat = 6,
ntyp = 1,
ecutwfc = 40.D0,
ecutrho = 480.D0,
occupations = 'smearing'
smearing ='mp',
degauss = 0.03,
nspin = 2,
starting_magnetization(1)= 0.003,
london=.true.,
/
&ELECTRONS
conv_thr = 1.D-6,
mixing_beta = 0.7D0,
diagonalization = "david",
/
&IONS
ion_dynamics="cg"
/
&CELL
cell_dynamics = 'damp-w',
press = 0.0,
/
ATOMIC_SPECIES
C 12.0107 C.pbe-rrkjus.UPF
ATOMIC_POSITIONS {angstrom}
C 0.00000000 0.00000000 0.00000000 1 1 0
C 0.00000000 1.41908472 0.00000000
C 0.00000000 0.00000000 3.15347111
C 11.22896342 0.70954236 3.15347111
C 0.00000000 0.00000000 6.30694222
C 0.00000000 1.41908472 6.30694222
K_POINTS {automatic}
4 4 1 1 1 1

Sincerely Yours
Masoud Nahali
SUT
_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://www.democritos.it/mailman/listinfo/pw_forum

--
Gabriele Sclauzero, EPFL SB ITP CSEA
PH H2 462, Station 3, CH-1015 Lausanne

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Gabriele Sclauzero

2010-07-13 11:02:15 UTC

Permalink

Post by Masoud Nahali
Dear Gabriele Sclauzero and pwscf Users
Many thanks for your attentions. The cell dimension and
the positions of the atoms are exactly correct.

Post by Masoud Nahali
I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It

Sincerely Yours
Masoud Nahali
SUT
_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://www.democritos.it/mailman/listinfo/pw_forum

--
Gabriele Sclauzero, EPFL SB ITP CSEA
PH H2 462, Station 3, CH-1015 Lausanne

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Masoud Nahali

2010-07-13 13:41:00 UTC

Permalink

Dear Gabriele and pwscf users

Perfectly I am sure that the positions of atoms is exactly correct. Note
that the input is belong to a (1*1) slab
and indeed you could not see a honeycomb ! but It is easy to see a honeycomb
when you have a (2*2) slab like this:

C 0.000352513 0.001055549 0.000000000
C 2.459305478 0.000933657 -0.008798962
C -1.229191611 2.130502759 -0.008802160
C 1.229819481 2.130516670 -0.008801637
C 0.000327447 1.420695799 -0.007291549
C -1.229173562 3.550193436 -0.007292113
C 2.459296954 1.420621766 -0.009715082
C 1.229852979 3.550208684 -0.007292392
C 0.000507119 0.000488868 3.187515573
C 2.459486429 0.000491165 3.187352189
C -1.228985445 2.130026298 3.187352283
C 1.229997454 2.130026366 3.187352273
C 1.229970852 0.710263879 3.187439366
C 0.000476982 2.839800509 3.187332530
C 3.688943914 0.710263968 3.187439418
C 2.459456202 2.839797012 3.187439363
C 0.000212609 0.000545761 6.403453823
C 2.459191836 0.000545600 6.403451859
C -1.229278051 2.130084159 6.403451942
C 1.229700861 2.130084205 6.403451921
C 0.000121431 1.420208410 6.403892505
C -1.229369239 3.549746424 6.403892711
C 2.459100653 1.420208253 6.403888354
C 1.229609673 3.549746469 6.403892772

a = 4.9178,
b = 4.9178,
c = 16.4112, 10 angstrom vacuum

Also, see the "As.vcs00.in" example in the "VCSexample" folder of quantum
espresso 4.2. In this example a vc-relax calculation
has been performed using the max_seconds and dt keyboard.

Many Thanks

I hope one helps these problems mentioned below.

Post by Gabriele Sclauzero
Are you completely sure? You can find as an attachment a picture (from
xcrysden) of the central graphene layer according to the positions given
in your input. You can now judge if it looks like a honeycomb lattice.
Yes, indeed the high ecutrho is important for ultrasoft
pseudopotentials. About the rippling : It was my mistake in selecting
a wrong pseudopotential which has a hole. Nicola had explained it
before and accordingly I solved it. The graphite surface is not
ripple. When I use the "max_second=6000 and dt=150" the job completes
very fast as the example of pwscf. Is the using of such keyboards
plausible?
Please spend some time reading Doc/INPUT_PW to understand the meaning of
the _keywords_. dt is used only for molecular dynamics runs, not
relaxations, while max_seconds has nothing to do with how fast your job
is completed.

Post by Masoud Nahali
Dear Gabriele Sclauzero and pwscf Users
Many thanks for your attentions. The cell dimension and the positions of
the atoms are exactly correct. Yes, indeed the high ecutrho is important

for

Post by Masoud Nahali
ultrasoft pseudopotentials. About the rippling : It was my mistake in
selecting a wrong pseudopotential which has a hole. Nicola had explained

Post by Masoud Nahali
before and accordingly I solved it. The graphite surface is not ripple.

When

Post by Masoud Nahali
I use the "max_second=6000 and dt=150" the job completes very fast as the
example of pwscf. Is the using of such keyboards plausible?
As I mentioned before I had used the optimized cell parameters of (1*1
slab) for vc-relaxing the (2*2 slab) and I expected to see the results

very

Post by Masoud Nahali
soon but the calculation was time consuming while there was only a very

very

Post by Masoud Nahali
bit change of the cell dimensions during this 16 hours. There is only 1-3
iterations per each step in the output file and each of them was time
consuming. The job was completed after about 50 steps.

-------------------------------------------------------------------------

Post by Masoud Nahali
Dear Masoud,
first I would suggest you to use bfgs as the algorithm for both ions

and

Post by Masoud Nahali
cell dynamics. Excepted particular cases, it should reach the minimum

much

Post by Masoud Nahali
faster.
Also, why do you specify the cell with such an unusual way. You simply

need

Post by Masoud Nahali
celldm(1) and celldm(3) with ibrav=4 if you want to describe an hexagonal
lattice. Other suggestions: your ecutrho looks really large to me, do you
really need it. On the other hand, degauss might be too large to describe

Post by Masoud Nahali
spin-polarized system.
Then, are you sure that you have built correctly your supercell? It looks
like there are some C-C bonds much shorter that others in the central
graphene plane (1.2 instead of 1.4 angs). Please check again your

structure.

Post by Masoud Nahali
In general, you can expect that if you relax the atoms in the supercell
some kind of surface-reconstruction may appear, since you leave more

freedom

Post by Masoud Nahali
to atoms to rearrange in structures with larger periodicity. I don't

think

Post by Masoud Nahali
this is the case for graphite, but you may find some ripples (as you
mentioned in your earlier emails, if I am not wrong) if the C-C bonds are

Post by Masoud Nahali
a distance shorter than the theoretical equilibrium distance (I guess).
HTH

------------------------------------------------------------------------------------------------
Dear Quantum Espresso Users

Post by Masoud Nahali
I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It takes 20

minutes with parallel running by 4 CPUs. Then I used the exact optimized
cell parameters (obtained from vc-relaxed calculation) to make a (2*2) slab
of graphite with 3 layers and I expected to see the results in a few
minutes. But amazingly it took 17 hours to complete. 48 steps were done in
the calculation for vc-relaxing the cell which have the parameters that had
been optimized before. The cell parameters only change a very bit in the
current vc-relaxing the (2*2) slab. I appreciate if one explain the
physical
procedure of vc-relaxing and the reason of the time needed for the
computation.

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Masoud Nahali

2010-07-12 18:24:30 UTC

Permalink

Dear Quantum Espresso Users

I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It takes 20
minutes with parallel running by 4 CPUs. Then I used the exact optimized
cell parameters (obtained from vc-relaxed calculation) to make a (2*2) slab
of graphite with 3 layers and I expected to see the results in a few
minutes. But amazingly it took 17 hours to complete. 48 steps were done in
the calculation for vc-relaxing the cell which have the parameters that had
been optimized before. The cell parameters only change a very bit in the
current vc-relaxing the (2*2) slab. I appreciate if one explain the physical
procedure of vc-relaxing and the reason of the time needed for the
computation.

input file:

CONTROL
calculation = "vc-relax",
pseudo_dir = "/home/koa/soft/qe4.2/
espresso-4.2/pseudo",
outdir = "/home/koa/tmp",
etot_conv_thr= 1.0D-4,
forc_conv_thr= 1.0D-3,
dt=80,
/
&SYSTEM
ibrav = 4,
a = 2.4579,
b = 2.4579,
c = 16.3069,
cosab = -0.5,
cosac = 1.0,
cosbc = 1.0,
nat = 6,
ntyp = 1,
ecutwfc = 40.D0,
ecutrho = 480.D0,
occupations = 'smearing'
smearing ='mp',
degauss = 0.03,
nspin = 2,
starting_magnetization(1)= 0.003,
london=.true.,
/
&ELECTRONS
conv_thr = 1.D-6,
mixing_beta = 0.7D0,
diagonalization = "david",
/
&IONS
ion_dynamics="cg"
/
&CELL
cell_dynamics = 'damp-w',
press = 0.0,
/
ATOMIC_SPECIES
C 12.0107 C.pbe-rrkjus.UPF
ATOMIC_POSITIONS {angstrom}
C 0.00000000 0.00000000 0.00000000 1 1 0
C 0.00000000 1.41908472 0.00000000
C 0.00000000 0.00000000 3.15347111
C 11.22896342 0.70954236 3.15347111
C 0.00000000 0.00000000 6.30694222
C 0.00000000 1.41908472 6.30694222
K_POINTS {automatic}
4 4 1 1 1 1

Sincerely Yours
Masoud Nahali
SUT
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Gabriele Sclauzero

2010-07-13 07:52:15 UTC

Permalink

? Gabriele Sclauzero, EPFL SB ITP CSEA
PH H2 462, Station 3, CH-1015 Lausanne

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Masoud Nahali

2010-07-13 10:49:53 UTC

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Dear Quantum Espresso Users

Post by Gabriele Sclauzero
I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It takes 20

Sincerely Yours
Masoud Nahali
SUT
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Masoud Nahali

2010-07-13 13:41:00 UTC

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for

Post by Masoud Nahali
ultrasoft pseudopotentials. About the rippling : It was my mistake in
selecting a wrong pseudopotential which has a hole. Nicola had explained

Post by Masoud Nahali
before and accordingly I solved it. The graphite surface is not ripple.

When

very

Post by Masoud Nahali
soon but the calculation was time consuming while there was only a very

very

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Post by Masoud Nahali
Dear Masoud,
first I would suggest you to use bfgs as the algorithm for both ions

and

Post by Masoud Nahali
cell dynamics. Excepted particular cases, it should reach the minimum

much

Post by Masoud Nahali
faster.
Also, why do you specify the cell with such an unusual way. You simply

need

structure.

Post by Masoud Nahali
In general, you can expect that if you relax the atoms in the supercell
some kind of surface-reconstruction may appear, since you leave more

freedom

Post by Masoud Nahali
to atoms to rearrange in structures with larger periodicity. I don't

think

Post by Masoud Nahali
this is the case for graphite, but you may find some ripples (as you
mentioned in your earlier emails, if I am not wrong) if the C-C bonds are

Post by Masoud Nahali
a distance shorter than the theoretical equilibrium distance (I guess).
HTH

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Dear Quantum Espresso Users

Post by Masoud Nahali
I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It takes 20

minutes with parallel running by 4 CPUs. Then I used the exact optimized
cell parameters (obtained from vc-relaxed calculation) to make a (2*2) slab
of graphite with 3 layers and I expected to see the results in a few
minutes. But amazingly it took 17 hours to complete. 48 steps were done in
the calculation for vc-relaxing the cell which have the parameters that had
been optimized before. The cell parameters only change a very bit in the
current vc-relaxing the (2*2) slab. I appreciate if one explain the
physical
procedure of vc-relaxing and the reason of the time needed for the
computation.

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