How to create vacuum slab in solvent region

Dear all,

I am trying insert a solid slab inside a solvent. To do this, I need to create a vacuum inside a solvent so that i can insert solid slab.

What are the possible ways to create vacuum inside solvent. I thought of using fix wall/lj93. Initially I will introduce wall with low sigma then gradually increase sigma till vacuum space is sufficient enough to insert slab.

Are there any better alternatives to this ?

Thanks in advance

RAVI KUMAR REDDY A

IISc Bangalore

Dear all,

I am trying insert a solid slab inside a solvent. To do this, I need to create a vacuum inside a solvent so that i can insert solid slab.

What are the possible ways to create vacuum inside solvent. I thought of using fix wall/lj93. Initially I will introduce wall with low sigma then gradually increase sigma till vacuum space is sufficient enough to insert slab.

Are there any better alternatives to this ?

how about first using a soft potential for the atoms to be inserted
with a height of 0 (i.e. no interaction). then use fix adapt to "grow"
the slab atoms in place (i.e. no time integration for them) and once
you have a sufficient space, you can switch the pair styles against
whatever you want and continue from there.

axel.

Dear axel,

My objective is to calculate free energy of inserting a slab in solvent. Proposed idea of using fix adapt is fine.

But if i use fix adapt, I was unable to track the energy contribution while growing slab because no information about this fix is written either to binary or global variables (thermo).

Editing fix adapt will help me in this case ?

RAVI KUMAR REDDY A

IISc Bangalore

Dear axel,

My objective is to calculate free energy of inserting a slab in solvent. Proposed idea of using fix adapt is fine.

But if i use fix adapt, I was unable to track the energy contribution while growing slab because no information about this fix is written either to binary or global variables (thermo).

Editing fix adapt will help me in this case ?

no. fix adapt only facilitates a change of force field parameters. to
determine free energies, you first have to figure out which specific
free energy method you want/can use in your case and then choose the
corresponding feature in LAMMPS. this means, you first need to read up
on the topic using text book material, and best first do some small
test calculations with known results.

axel.

Is your solvent homogeneous?
If it is, you can create molecule file for solvent and slab , create_atoms for slab in someplace, define region to create_atoms for solvent as everything except the slab place maybe, then relax system with minimize or soft potentials…

Dear Axel,

I resolved my issue by editing pair_style.born.cpp file. which can work like a one dimensional field. I used only first part of the potential ( i.e. C and D to be zero ) to give pure repulsive potential. depending on the strength of sigma i can create the vacuum in the middle of the solvent, then i can insert my solid slab.

thanks for your help…