VASP_BOND_FIX

Fixing bond length for specific atom dimers during geometry optimisation (relaxation) process implemented in VASP package.

Method

For symmetrical dimer For asymmetrical dimer, mass weighted average forces are used

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 centre (POS_1+POS_2)/2.

Caveats

1. Initial bond length should in general equal the one specified in the CONSTRAIN file.
2. Convergence maybe slow, you 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, you should have a VASP license to use this patch.

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.