VASP_BOND_FIX

Fixing bond length for specific atoms during geometry optimization (relaxation) process implemented in VASP package.

Method

implementation

loop over NSTEP:
  |--> scf force
  |--add-->  constrain force (change force)
  |--> Force convergence?
  |--> IBRION ionic movements
  |--add-->  constrain distance (change position)
end loop

Check stdout for CONSTRAIN_FORCE is done. and CONSTRAIN_POSITION is done. Check OUCAR for CONSTRAINED TOTAL-FORCE block.

Install

Put bond_length.patch file in the root directory of your VASP distro and type:

$ patch -p0 < bond_length.patch

Usage

An additional file called CONSTRAIN is needed for specifying which bonds to fix, and its length. syntax:

ATOM1# ATOM2# BOND_LENGTH1
ATOM3# ATOM4# BOND_LENGTH2
...

Then a regular relaxation can be performed.

if Selective-dynamics on one of the constrained atoms, the bond length constrain will move the other atom, if no Selective-dynamics used on the constrained atoms, the bond length is distributed evenly to each atom according to the bond center (POS_1+POS_2)/2.

Caveats

1. Initial bond length should equal the one specified in CONSTRAIN file.
2. Convergence maybe slow, might want to loosen the convergence criteria
3. Be careful with variable cell calculation, we do not apply constrains on cell stress.

Notes

1. No source file is shared.

2. The correctness of this patch has been checked with few simple cases.

3. Any result obtained by this fix should be carefully checked by yourself.

4. This patch only works on currently VASP 5.4.4.

5. A major part of this implementation was contributed by someone at GaTech.