[gmx-users] Question about OPLS atomtypes

[Andrew DeYoung]

Hi, 

If you have time, I have a question about the OPLS parameters given in the
files located in the "top" directory (for example,
/usr/local/gromacs/share/gromacs/top/oplsaa.ff).

In the oplsaa.ff directory, there is a file atomtypes.atp.  At the top of
this file, it says: 

; Types 1-134 are from the united-atom OPLS, which can be
; useful for solvents and/or CH2 optimizations (e.g. in lipids).
; Explicit all-atom parameters start with opls_135.

Is it true that there _could_ be all-atom parameters in Types 1-134, the
united-atom section?

For example, I am trying to find parameters for carbon tetrachloride, CCl4.
I have found these possibilities in atomtypes.atp:

 opls_122   12.01100  ; C CCl4
 opls_123   35.45300  ; Cl CCl4

which, for example, have the following entries in ffnonbonded.itp:

 opls_122   CT	6     12.01100     0.248       A    3.80000e-01  2.09200e-01
 opls_123   Cl	17     35.45300    -0.062       A    3.47000e-01
1.11295e+00

Clearly, these entries each correspond to individual atoms, even though
opls_122 and opls_123 are in the "united-atom OPLS" section.  I will be
describing the remainder of my system using all-atom OPLS parameters (CCl4
is just the solvent; I will also have solute); is it likely reasonably safe
to use the above parameters for C and Cl?  In other words, I would be
"mixing" parameters from the united-atom and all-atom sections, because I
have been unable to find all-atom parameters for C and Cl in CCl4.

Thank you!

Andrew DeYoung
Carnegie Mellon University