[gmx-users] in vacu simulation

[Mark Abraham]

On 16/02/2012 2:08 PM, Juliette N. wrote:
>
>
> On 15 February 2012 21:00, Mark Abraham <Mark.Abraham at anu.edu.au 
> <mailto:Mark.Abraham at anu.edu.au>> wrote:
>
>     On 16/02/2012 12:22 PM, Justin A. Lemkul wrote:
>
>
>
>         Juliette N. wrote:
>
>             Hi all,
>
>             I am trying to run simulation in vaccum using the the
>             changes shown below to the usual mdp file.
>             pbc              =  no
>
>             ;coulombtype         =  PME   ;vdw-type            =  Shift
>             ;        Cut-offs
>             rlist               =  0   rcoulomb     =  0
>             rvdw              =  0
>
>             nstlist             =  0               ns_type          =
>              simple
>
>             Can anyone help me with some short questions please?
>
>             1- for pbc=no, I need to comment
>
>             ;coulombtype         =  PME   ;vdw-type            =  Shift
>             so it defaults to vdw-type =  Cut-off which are not
>             suitable algorithms. Is using cut offs justified for in
>             vacu runs?
>
>
>         Plain truncations in condensed-phase systems lead to
>         artifacts.  Neither of those conditions apply here, as you're
>         using infinite cutoffs.
>
>
>             2- I am not clear about using infinite cutoffs. Why one
>             refers to infinite cutoffs when
>
>             rlist               =  0   rcoulomb     =  0
>             rvdw              =  0     ?
>
>             My understanding is that this settings means zero cutoff
>             i.e no interaction is calculated. Why does this setting
>             refer to infinite rc?
>
>
>
>         That's the way the code works.  There are various parameters
>         that can be set to -1, for instance, and that doesn't mean
>         quantities are calculated every -1 steps ;)
>
>         Setting cutoffs to zero in this manner mean *all* interactions
>         are calculated, not none.  Prove it to yourself with a
>         zero-step MD run.  The nonbonded energy terms will not be
>         zero, as they would in the case that no interactions would be
>         calculated.
>
>
>     Or read about pbc=no in manual section 3.4.9 like I suggested
>     Juliette do earlier this week...
>
>
> Thanks Justin and Mark. I think you meant 7.3.9 which I did when you 
> referred me to that. My problem was that I did not expect rc=0  is 
> *just defined* as infinite cutoff in gromacs. To me rc=0 looked more 
> equivalent to no interaction than infinite cutff off (all interactions).

Sure, but reading the documentation is usually a better idea than making 
assumptions :)

The underlying reason for this behaviour is that it is much easier for 
the person writing the code to have one parameter that occasionally has 
a "special" meaning when it takes a nonsense value (like rc<=0) then it 
is to have a slew of parameters that have to be managed when they are 
input (and checked, and documented) and then possibly passed through a 
cascade of functions (lots of bureaucracy and chances to make errors) 
before they are used. The alternative costs the programmer more time. In 
an ideal world there would be an infinite amount of such time, but given 
the amount most people are prepared to pay for scientific software, that 
time is severely limited.

> And also I dont see why do we need to change rc to infinite. I mean if 
> force fields dictate cutoffs based on a distance where nonbonded 
> interactions are close enough to zero (negligible), what  purpose use 
> of infinite cutoff serve?

Efficiency, like I said in the first post in this thread. Given that 
your force field was parametrized with given cut-offs for the condensed 
phase, to what purpose do you wish to calculate in vacuo? The 
perturbation from calculating in vacuo will be much larger than the 
perturbation from the use of infinite cut-offs.

Mark
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