# [lammps-users] Need your help for ibar parameter in MEAM potential in Lammps!!

Hi, dear LAMMPers and EAMers:

In command pair_style meam, it has 19 parameters: elt, lat, z, ielement, atwt, alpha, b0, b1, b2, b3, alat, esub, asub, t0, t1, t2, t3, rozero, ibar.
I know the meaning of all the parameters but last one ‘ibar’. It seems that it does not show up in MEAM potential formulas.

Best Regards

Rongguang

ibar determines the model of the background density that goes with the material that you are simulating.

This is the DYNAMO function for rhobar:

c************************************************************************
c this routine calculates unscaled rhobar
c
function bar(rho0,A,ibar,z,dang1,dang2)
implicit real*8 (a-h,o-z)
data x0,n/-0.9,9/
c n=-x_0/(1+x_0)
c rhobar = rho0 * ang
c derivative of angular term wrt rho0 and A the angular density
common /err/ ibarer,ibare
data ibarer/0/
cprint *,'in bar ',rho0,A,ibar,z
bar=1e-20
if(rho0.le.0.0) return
ib=abs(ibar)
if (ib.eq.0 .or. ib.eq.4) then
c ang = sqrt(1+A/rho0^2)
ang = (1+A/rho0**2)
if(ang.lt.1+x0) then
x=ang-1
ang=sqrt((1+x0)*(x0/x)**n)
dang1=ang*n/rho0
dang2=-ang*n/(2*A)
ibare=ibare+1
ibarer=ibarer+1
cprint *,'in bar ',rho0,A,x0
else
ang=sqrt(ang)
dang1 = -A/(rho0**3*ang)
dang2 = 1/(2*rho0**2*ang)
endif
elseif (ib.eq.1) then
c ang = exp(0.5 A/rho0^2)
ang = exp(0.5*A/rho0**2)
dang1 = -ang*A/rho0**3
dang2 = ang/(2*rho0**2)
elseif (ib.eq.2) then
c ang = exp(0.5 A/z^2)
ang = exp(0.5*A/z**2)
dang1 = 0
dang2 = ang/(2.0*z**2)
elseif (ib.eq.3) then
c ang = 2/(1+exp(-A/rho0^2)
ex=exp(-A/rho0**2)
ang = 2.0/(1.0+ex)
dang1 = -ang**2*ex*A/rho0**3
dang2 = ang**2/2.0*ex/rho0**2
endif
bar=rho0*ang
cprint *,'leaving bar',bar
return
end

There must be something analogous in LAMMPS.

Steve Valone