Hi,

I got some problems when I calculated the volecity autocorrelation coefficient.

Here is my input file

# Si EMD test

units metal

dimension 3

boundary p p p

atom_style atomic

neighbor 1.0 bin

neigh_modify delay 5 check yes

lattice custom 5.431 a1 1.0 0.0 0.0 a2 0.0 1.0 0.0 a3 0.0 0.0 1.0 &

basis 0.0 0.0 0.0 basis 0.0 0.5 0.5 basis 0.5 0.0 0.5 basis 0.5 0.5 0.0 &

basis 0.25 0.75 0.75 basis 0.75 0.25 0.75 basis 0.75 0.75 0.25 basis 0.25 0.25 0.25

region box block 0.0 200.0 0.0 8.0 0.0 4.0

create_box 1 box

create_atoms 1 box basis 1 1 basis 2 1 basis 3 1 basis 4 1 basis 5 1 basis 6 1 basis 7 1 basis 8 1

mass 1 28.085

velocity all create 300.0 43454 dist gaussian mom yes

fix 1 all nvt temp 300.0 300.0 0.005

pair_style sw

pair_coeff * * SiGe.sw Si

neighbor 0.3 bin

neigh_modify every 5 delay 0 check no

compute 2 all vacf

fix 5 all vector 1 c_2[4]

variable diff equal dt*trap(f_5)

thermo_style custom step v_diff

thermo 10

timestep 0.0003

fix 3 all print 10 “${diff}” file vacf.txt

run 10000

clear

I used the input file above, and got the result that I thought was wrong.

My question is that I want to calculate volecity autocorrelation coefficient by using the formula

But, the command vacf seems like not use this formula.

It seems like that LAMMPS calculate vacf as

Not use v=v(t).

Is this understanding correct?

And if I want to calculate as the definition of first equation, is there any command in LAMMPS that I can use?

Thank a lot,

Michael I-Ta Hsieh