On 14/02/2012 6:26 AM, Juliette N. wrote:
Hi all,
I have two simple question on in vacu calculations.
1- Say we start from rc=1, and go all the way to rc=0.1 then we say
cut off has been reduced from 1 nm to 0.1 nm meaning that atoms fall
within 0.1 nm interact with each other. Now when we set rc=0, why dont
we consider this as zero cutoff and that No interaction is cacluated
for in vacu simulation. When one sets r_vdw= 0, r_coulomb=0, why
people call this infinite cutoff and not zero cutoff (=no nonboded
interaction calculated).
Because it is defined that way. See manual section 7.3.9. In theory, you
could have another .mdp variable "cut-offs = yes/no" to serve this
purpose, but since the value zero serves no useful purpose, it is used
in the above way.
2-For calculation of heat of vaporisation why do we need single
molecule in vacu? why cant we leave cut off as in liquid phase?
3- Also why do we turn off PBC for in vacu runs?
Usually efficiency. Finding the nearest neighbours takes time, and if
there are few atoms at all (as is often the case with vacuum
simulations) it can be cheaper just to compute all the interactions.
Mark
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