[gmx-users] problem going from cut-off to pme

[Yiannis]

Hello,
I have a strange problem with a 100 molecule system where I used cut- 
off electrostatics and I want to use pme from now.

After md with Coulomb cut-off, I minimised with steep (and the same  
Coulomb cut-off) and it ends OK:

> Step=  395, Dmax= 1.7e-06 nm, Epot= -5.04260e+04 Fmax= 1.15989e+02,  
> atom= 4130
> Step=  396, Dmax= 2.0e-06 nm, Epot= -5.04045e+04 Fmax= 1.15977e+02,  
> atom= 4130
> Stepsize too small, or no change in energy.
> Converged to machine precision,
> but not to the requested precision Fmax < 1
>
> Double precision normally gives you higher accuracy.
>
> writing lowest energy coordinates.
>
> Steepest Descents converged to machine precision in 397 steps,
> but did not reach the requested Fmax < 1.
> Potential Energy  = -5.0425977e+04
> Maximum force     =  1.1598901e+02 on atom 4130
> Norm of force     =  1.6355400e+03

I take this minimised .gro and I minimise it again but now I change  
to PME electrostatics. It crashes from the beginning with an infinite  
force (not on the atom with the previous maximum force) and the  
energy is huge: 6.56504e+19 instead of -5.0425977e+04 which was the  
end of the previous minimisation. Do you have any explanation? Should  
I change some PME parameters?

> Step=    0, Dmax= 1.0e-02 nm, Epot=  6.56504e+19 Fmax=         inf,  
> atom= 164
> Step=   14, Dmax= 1.2e-06 nm, Epot=  6.56504e+19 Fmax=         inf,  
> atom= 164
> Stepsize too small, or no change in energy.
> Converged to machine precision,
> but not to the requested precision Fmax < 1
>
> Double precision normally gives you higher accuracy.
>
> writing lowest energy coordinates.
>
> Steepest Descents converged to machine precision in 15 steps,
> but did not reach the requested Fmax < 1.
> Potential Energy  =  6.5650353e+19
> Maximum force     =            inf on atom 164
> Norm of force     =            inf

Thanks for any advice,

Ioannis