[gmx-users] Problem with OPLS parameters (too few parameters on line)

Parul tew parultew at gmail.com
Wed Apr 17 10:28:24 CEST 2013

Dear GMX users,

I am working on a protein which I want to simulate in a lipid bilayer
environment (POPC) and want to use OPLS force field for the same. I
followed the chris neale  directives to make the necessary changes i.e.
sigma and epsilon, I incorporated the modified atom types of lipids in
LO    LO    1    15.9994    0    A    0.29599993    0.87869434
 ;carbonyl O, OPLS"
LOM    LOM    1    15.9994    0    A    0.29599993    0.87869434
 ;carboxyl O, OPLS

I also made changes to the [pair types] and appended them in the
oplsaanb_lipid.itp file
[ pair types ]
; i    j    func    sigma    epsilon
LO    LO    1    0.29603763    0.10964147
LO    LOM    1    0.29603763    0.10964147
LO    opls_116    1    0.306316902    0.094663158

and also changed the OW to opls_116 and removed HW lines.
Now I want to generate the tpr file using grompp where I had generated a
topol_popc.top :
; Include chain topologies
#include "oplsaa_lipid.ff/forcefield.itp"
#include "popc.itp"

; Include water topology
#include "oplsaa_lipid.ff/spc.itp"

; System specifications
[ system ]
128-Lipid POPC Bilayer

[ molecules ]
; molecule name nr.
POPC 128
SOL 2460
Now when I run the grompp command it gives me following error:
ERROR 1 [file ffoplsaanb_lipid.itp, line 852]:
  Invalid directive pair types

WARNING 1 [file ffoplsaanb_lipid.itp, line 854]:
  Too few parameters on line (source file toppush.c, line 246)
Generated 359128 of the 359128 non-bonded parameter combinations
Generating 1-4 interactions: fudge = 0.5
Generated 359128 of the 359128 1-4 parameter combinations
Excluding 3 bonded neighbours molecule type 'POPC'

There were 91 warnings

Program grompp, VERSION 4.5.5
Source code file: grompp.c, line: 1372

Fatal error:
There was 1 error in input file(s)

I am not able to get where I am going wrong.
Thanking you,

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