关注：

1） POTIM、ISIF参数的含义

2） POTIM、ISIF的配合使用保持对称性不变

Thanks to  Xinqiu Chen and Xiyue Li

1. POTIM取大一点，仍采用ISIF=3

2.  分两步优化，

(1) 在每个压力点下优化晶格常数，保持原子位置不动     ISIF=7

(2) 在每个压力点下优化原子位置，保持晶格常数不动     ISIF=2

或许可解决对称性自动转化问题

The following table shows the meaning of ISIF.  At the moment cell changes are only supported for  relaxations and nor fot molecular dynamics simulations.

ISIF	calculate	calculate	relax	change	change
	force	stress tensor	ions	cell shape	cell volume
0	yes	no	yes	no	no
1	yes	trace only	yes	no	no
2	yes	yes	yes	no	no
3	yes	yes	yes	yes	yes
4	yes	yes	yes	yes	no
5	yes	yes	no	yes	no
6	yes	yes	no	yes	yes
7	yes	yes	no	no	yes

Trace only means that only the total pressure, i.e. the line

（一) POTIM参数解释：

POTIM= [real]

In case IBRION=0 (MD) , POTIM specifies the time step in fs. For IBRION=1,2 or 3, POTIM serves as a scaling constant for the forces.

POTIM supplies the time step  for an ab-initio molecular dynamics (IBRION=0),

and must be entered by the user for all MD simulations.

In addition POTIM severs as a ``scaling constant'' in all minimization algorithms (quasi-Newton, conjugate gradient, and damped molecular dynamics).  Especially the Quasi-Newton algorithm is sensitive to the choice of this parameter

(二) IBRION解释

IBRION=2

A conjugate-gradient algorithm (a simple discussion of this algorithm

can be found for instance in [28]) is used to relax the ions into their instantaneous groundstate.

In the first step ions (and cell shape) are changed along the direction of the steepest descent (i.e. the direction of the calculated forces and stress tensor).

The conjugate gradient method requires a line minimization, which  is performed in several steps:

i) first  a trial step into the search direction (scaled gradients) is done, with the length of the trial step controlled by the POTIM parameter (section 6.23). Then the energy and the forces are recalculated.

ii) The approximate minimum of the total energy is calculated from a cubic  (or quadratic) interpolation  taking into account the change of the total energy and the change of the forces (3 pieces of information), then a corrector step to the approximate minimum is performed.

iii) After the corrector step the forces and energy are recalculated and  it is checked whether the forces  contain a significant component parallel to the previous search direction. If this is the case,  the line minimization is improved by further corrector steps using a variant of Brent's algorithm[28].

To summarize: In the first ionic step the forces are calculated for the initial configuration read from POSCAR , the second step is a trial (or predictor step), the third step is a corrector step. If the line minimization is sufficiently accurate in this step,  the next trial step is performed.

##################ISIF=7##################

cp  cont-120g POSCAR

for i in  1200 1100 1000 900 800

do
cat > INCAR << EOF
SYSTEM = Various- local optimisation
PREC = Accurate
ENCUT = 500.0
EDIFF = 1E-6
EDIFFG = -2E-3
#SYMPREC=1e-3
IBRION = 2
POTIM = 0.05
ISIF = 7
NSW = 100

#LSDA-plus-U
ISPIN=2
LDAU = .TRUE.
LDAUTYPE = 2
LDAUL = -1  3
LDAUU = 0.00 4.10
LDAUJ = 0.0   0.1
LDAUPRINT = 0
LMAXMIX = 6

PSTRESS = $i
ISMEAR = 1
SIGMA = 0.2
#ISYM=0
#LREAL = .FALSE.
LCHARG = FALSE
LWAVE = FALSE
EOF

# start calculation

$PARA -n $NP -machinefile .nodelists.$$ $EXEC
 
cp CONTCAR CONTCAR_$i-01
cp OSZICAR OSZICAR_$i-01
cp OUTCAR OUTCAR_$i-01
cp STDOUT STDOUT_$i-01
cp CONTCAR POSCAR

# 2nd
$PARA -n $NP -machinefile .nodelists.$$ $EXEC

cp CONTCAR CONTCAR_$i-02
cp OSZICAR OSZICAR_$i-02
cp OUTCAR OUTCAR_$i-02
cp STDOUT STDOUT_$i-02
cp CONTCAR POSCAR

#3rd
$PARA -n $NP -machinefile .nodelists.$$ $EXEC

cp CONTCAR CONTCAR_$i-03
cp OSZICAR OSZICAR_$i-03
cp OUTCAR OUTCAR_$i-03
cp STDOUT STDOUT_$i-03
cp CONTCAR POSCAR

#4th
$PARA -n $NP -machinefile .nodelists.$$ $EXEC

wait

cp CONTCAR POSCAR
mv  CONTCAR CONTCAR_$i-04
mv  OUTCAR OUTCAR_$i-04
mv OSZICAR OSZICAR_$i-04
mv STDOUT STDOUT_$i-04
done

###############ISIF=2#############

for i in  1200 1100 1000 900 800

do
cat > INCAR << EOF
SYSTEM = Various- local optimisation
PREC = Accurate
ENCUT = 500.0
EDIFF = 1E-6
EDIFFG = -2E-3
#SYMPREC=1e-3
IBRION = 2
POTIM = 0.05 【是否太小】
ISIF = 2
NSW = 100

#LSDA-plus-U
ISPIN=2
LDAU = .TRUE.
LDAUTYPE = 2
LDAUL = -1  3
LDAUU = 0.00 4.10
LDAUJ = 0.0   0.1
LDAUPRINT = 0
LMAXMIX = 6

PSTRESS = $i
ISMEAR = 1
SIGMA = 0.2
#ISYM=0
#LREAL = .FALSE.
LCHARG = FALSE
LWAVE = FALSE
EOF

cp CONTCAR_$i-04 POSCAR
# start calculation

$PARA -n $NP -machinefile .nodelists.$$ $EXEC
 
cp CONTCAR CONTCAR_$i-01
cp OSZICAR OSZICAR_$i-01
cp OUTCAR OUTCAR_$i-01
cp STDOUT STDOUT_$i-01
cp CONTCAR POSCAR

# 2nd
$PARA -n $NP -machinefile .nodelists.$$ $EXEC

cp CONTCAR CONTCAR_$i-02
cp OSZICAR OSZICAR_$i-02
cp OUTCAR OUTCAR_$i-02
cp STDOUT STDOUT_$i-02
cp CONTCAR POSCAR

#3rd
$PARA -n $NP -machinefile .nodelists.$$ $EXEC

cp CONTCAR CONTCAR_$i-03
cp OSZICAR OSZICAR_$i-03
cp OUTCAR OUTCAR_$i-03
cp STDOUT STDOUT_$i-03
cp CONTCAR POSCAR

#4th
$PARA -n $NP -machinefile .nodelists.$$ $EXEC

wait

cp CONTCAR POSCAR
mv  CONTCAR CONTCAR_$i-04
mv  OUTCAR OUTCAR_$i-04
mv OSZICAR OSZICAR_$i-04
mv STDOUT STDOUT_$i-04
done