AMBER Archive (2004)

Subject: Re: AMBER: Ewald parameter range

From: David A. Case (case_at_scripps.edu)
Date: Fri Jul 23 2004 - 03:53:50 CDT

On Thu, Jul 22, 2004, Johnson Agbo wrote:

> I am running minimization and dynamics on cubic ice with periodic
> boundary conditions,

Amber has limitations in simulating small unit cells. Basically, the
cutoff + "skin" has to be less than half of the unit cell size.

>
> Ewald PARAMETER RANGE CHECKING:
> parameter skinnb+cutoffnb: (nonbond list cut) has value 0.10000E+02
> This is outside the legal range
> Lower limit: 0.00000E+00 Upper limit: 0.95370E+01

So, you need to use a smaller cutoff + skin (probably by reducing skinnb
from its default value of 2 to a value of 1), or create a bigger unit cell.

> I went into the program file ( ew_legal.h ) and changed the
> coefficients to the values shown below,
> ( _REAL_ ew_coefflo,ew_coeffhi
> parameter(ew_coefflo=0.0d0,ew_coeffhi=17.d0)

Your problem has really nothing to do with the ewald coefficeint....

...dac

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