[gmx-users] OPLS benzene issue

[Jeffery Perkins]

Hello Gromacs users,
I've been trying to run a system of benzene at 298 K and 1 bar as a test for future use as a solvent
and have built it in OPLS using opls_145 for the carbons and opls_146 for the hydrogens, these
are listed in the ffoplsaa files as benzene parameters.  However I've found that over a reletively
short run the system is apparently freezing into a solid (density goes to 1000 when it should be
around 800) but both the temperature and pressure look like what I'm coupling to.

I'm not sure if I've made a huge error in something or what is going on, I'll include my itp file here:

[ moleculetype ]
Ben	3

[ atoms ]
1	opls_145   1	Ben	C	1   -0.115   12.011   
2	opls_145   1	Ben	C	2   -0.115   12.011   
3	opls_145   1	Ben	C	3   -0.115   12.011   
4	opls_145   1	Ben	C	4   -0.115   12.011   
5	opls_145   1	Ben	C	5   -0.115   12.011   
6	opls_145   1	Ben	C	6   -0.115   12.011
7	opls_146   1  Ben	H	1   0.115	1.00800
8	opls_146   1  Ben	H	2   0.115	1.00800
9	opls_146   1  Ben	H	3   0.115	1.00800
10	opls_146   1  Ben	H	4   0.115	1.00800
11	opls_146   1  Ben	H	5   0.115	1.00800
12	opls_146   1  Ben	H	6   0.115	1.00800

[ bonds ]
1       2       1
1       6       1
1       7       1
2       3       1
2       8       1
3       4       1
3       9       1
4       5       1
4       10      1
5       6       1
5       11      1
6       12      1

[ angles ]
1       2       3       1
1       2       8       1
1       6       5       1
1       6       12      1
2       1       6       1
2       1       7       1
2       3       4       1
2       3       9       1
3       2       8       1
3       4       5       1
3       4       10      1
4       3       9       1
4       5       6       1
4       5       11      1
5       4       10      1
5       6       12      1
6       1       7       1
6       5       11      1

[ dihedrals  ]
1       2       3       4       3
1       2       3       9       3
1       6       5       4       3
1       6       5       11      3
2       1       6       5       3
2       1       6       12      3
2       3       4       5       3
2       3       4       10      3
3       2       1       6       3
3       2       1       7       3
3       4       5       6       3
3       4       5       11      3
4       3       2       8       3
4       5       6       12      3
5       4       3       9       3
5       6       1       7       3
6       1       2       8       3
6       5       4       10      3
7       1       2       8       3
7       1       6       12      3
8       2       3       9       3
9       3       4       10      3
10      4       5       11      3
11      5       6       12      3

and my mdp file(edited out some comments that aren't important):

nsteps			 = 200000


; Output frequency for xtc file = 
nstxtcout                = 500

;Output frequency for coords (x), velocities (v) and forces (f) = 
nstxout                  = 500
nstvout                  = 500
nstfout                  = 0

; Output frequency for energies to log file and energy file = 
nstenergy                = 500
energy_grps = Ben 

; Temperature(K) & Pressure(bar)
gen_temp                 = 298.15 
ref_t                    = 298.15
comm_mode = None

;Not all things will use xtc output, but if we define it here,
;then the other files should never need changing.
xtc_grps                 = Ben
coulombtype              = Cut-off


;Pressure (in units: bar)
ref_p                    = 1


;;;OUTPUT;;;

;Output frequency for coords (x), velocities (v) and forces (f) = 
nstxout                  = 5000
nstvout                  = 5000
nstfout                  = 0
; Output frequency for energies to log file and energy file = 
nstlog                   = 0
nstenergy                = 50000

;;;TEMPERATURE;;;
;generation is disabled, but gen_temp must be set for files importing this one
gen_vel                  = yes
gen_seed                 = -1
tau_t                    = 0.1
Tcoupl                   = Berendsen
tc-grps                  = System
;;;PRESSURE;;;
Pcoupl                   = Berendsen
tau_p                    = 0.1
compressibility          = 4.5e-5

; These are all = 1 by default, so they could possibly be left out
rlist				 = 1.5
rcoulomb			 = 1.5
rvdw				 = 1.5
vdwtype			 = Cut-off
coulombtype			 = Cut-off

; Output frequency for all pair energies (can make LARGE files) (0=never)
nstdisreout              = 0

; Apply long range dispersion corrections for Energy and Pressure
; We might not want this at all, default is = no
DispCorr                 = EnerPres
;steps between neighbor list updates = 
nstlist                  = 5
;Something to do with energy minimization...?
emtol                    = 100

; Non-equilibrium MD stuff
;freezegrps               = 
;freezedim                = 
;energygrp_excl:		 = DRG DRG
; NMR refinement stuff
; Distance restraints type: No, Simple or Ensemble
;disre                    = No
;disre                    = Simple
; Force weighting of pairs in one distance restraint: Conservative or Equal
;disre-weighting          = Conservative
disre-weighting          = Equal
; Use sqrt of the time averaged times the instantaneous violation
;disre-mixed              = no
;disre-fc                 = 1000
;disre-tau                = 0
; Output frequency for pair distances to energy file

I've tried several things including pressurizing to 1000 bar then releasing it to 1 bar and
have had no success.

Any suggestions would be much appreciated,

Jeffery Perkins