Hi

I have seen the example for elastic constants at 0K in lammps examples

I would like to look at the elastic properties at various temperatures such as 300k, 500K and so on.

can you tell what modification to be made in the example script?

Hi

I have seen the example for elastic constants at 0K in lammps examples

I would like to look at the elastic properties at various temperatures such as 300k, 500K and so on.

can you tell what modification to be made in the example script?

Hi,

usually the calculation of the elastic constants can be donde by

different methods. First is a method that uses the fluctuation formula

at the finite temperature "J. R. Ray, Comp. Phys .Rep. 8 (1988),109"

. The other is the method that takes into account the internal

displacement in the range of the harmonic approximation J. W. Martin,

J. Phys. C, 8 (1975), 2858. At 0K the calculation of the elastic

constant is something trivial i.e just take the derivative of the

stress vs the displacement cijkl = sigma_ij/epsilon_kl. (dont forget

set the forces to zero). However for a finite pressure or temperaure

thing turns ugly ...

Oscar G.

Oscar is correct I believe, and Aidan may want to

comment on "ugly" ...

Steve

Yes, in theory the strain fluctuation formula can be used with NPT MD

sampling, but it practice it does not work. The stress fluctuation

counterpart does work, but it requires sampling second derivatives of

energy w.r.t. strain, while LAMMPS only computes.first derivatives i.e.

stress. The standard solution is to sample stress in NVT and N(V+epsilon)T

MD, where epsilon represents as small deformation. You have to run 6

different deformations to get all the elastic constants, or 12 if you do

both positive and negative deformations. That's pretty ugly. There is a

better method developed by Yubao Zhen[1] that is on our list of things to

do. Maybe in a few months.

Aidan

[1] Yubao Zhen ＊, Chengbiao Chu, A deformation–fluctuation hybrid method

for fast evaluation of elastic constants with many-body potentials,

Computer Physics Communications 183 (2012) 261–265,