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# H2O

## Contents

Relaxation of an ${\displaystyle {\mathrm {H}}_{{2}}{\mathrm {O}}}$ molecule.

## Input

### POSCAR

H2O _2
0.52918   ! scaling parameter
15 0 0
0 15 0
0 0 15
1 2
select
cart
0.00     0.00     0.00 F F F
1.10    -1.43     0.00 T T F
1.10     1.43     0.00 T T F


All coordinates are scaled by the factor 0.52918.

### INCAR

PREC = Normal    ! standard precision
ENMAX = 400      ! cutoff should be set manually
ISMEAR = 0 ; SIGMA = 0.1
IBRION = 1       ! use DIIS algorithm to converge
NFREE = 2        ! 2 independent degrees of freedom
NSW = 10         ! 10 ionic steps
EDIFFG = -0.02   ! forces smaller 0.02 A/eV


### KPOINTS

Gamma-point only
0
Monkhorst Pack
1 1 1
0 0 0


## Calculation

• Use PREC=Normal (Default for VASP.5.X)
• It is strongly urged that the energy cutoffs are set manually in the INCAR file, as it provides more control over the calculations.
• For the ionic optimization the DIIS algorithm is used. This algorithm builds an approximation of the Hessian matrix and converges usually faster than the conjugate gradient algorithm. It is however recommended to set the independent degrees of freedom manually. *EDIFFG determines when to terminate the relaxation. Positive values: energy change between steps must be less than the value set by EDIFFG. Negative values: ${\displaystyle |{\mathbf {F}}_{{i}}|<|{\mathrm {EDIFFG}}|\forall i=1,N_{{{\mathrm {ions}}}}}$.