Dear All
I would like to perform MD simulations with benzene as a solvent and
am observing some strange behaviour when I use Lincs to constrain all
the bonds. When I run a fully flexible NPT MD of a box of 320 benzene
molecules simulation at 298K and 1bar, the density comes out at 841
g/l, not too far away from the experimental value of 876 g/l. However
when I constrain all bonds using Lincs, the system expands rapidly,
stabilising at a density of 2.62 g/l ! Both simulations started from
NVT equilibrated simulations fixed to the experimental density. In the
papers describing OPLS parametrisation, the MC simulations were indeed
performed with fully flexible molecules, but it surprises me that the
bond constraints would affect the density so strongly. Does anyone
have any ideas why this is occurring? I am using a cutoff of 1.5 nm
for VDW and Coulomb interactions without EWALD/PME but this is
consistent with how OPLS was parametrised. There are no Lincs warnings
in the log file of the constrained simulation.
Please find my mdp and Benzene itp files below, the only difference
between the flexible and constrained runs being dt = 0.001/0.002 and
constraints = none/all-bonds respectively.
Many thanks for your ideas/explanations as to what could be going on,
Mike
;
; File 'mdout.mdp' was generated
; By user: mwykes (7017)
; On host: node168
; At date: Thu Feb 4 20:05:46 2010
;
; VARIOUS PREPROCESSING OPTIONS
; Preprocessor information: use cpp syntax.
; e.g.: -I/home/joe/doe -I/home/mary/hoe
include =
; e.g.: -DI_Want_Cookies -DMe_Too
define = -DFLEX_SPC
; RUN CONTROL PARAMETERS
integrator = md
; Start time and timestep in ps
tinit = 0
dt = 0.002
nsteps = 5000000
; For exact run continuation or redoing part of a run
; Part index is updated automatically on checkpointing (keeps files separate)
simulation_part = 1
init_step = 0
; mode for center of mass motion removal
comm-mode = LINEAR
; number of steps for center of mass motion removal
nstcomm = 1
; group(s) for center of mass motion removal
comm-grps =
; LANGEVIN DYNAMICS OPTIONS
; Friction coefficient (amu/ps) and random seed
bd-fric = 0
ld-seed = 1993
; ENERGY MINIMIZATION OPTIONS
; Force tolerance and initial step-size
emtol = 1.0
emstep = 0.1
; Max number of iterations in relax_shells
niter = 20
; Step size (ps^2) for minimization of flexible constraints
fcstep = 0
; Frequency of steepest descents steps when doing CG
nstcgsteep = 1000
nbfgscorr = 10
; TEST PARTICLE INSERTION OPTIONS
rtpi = 0.05
; OUTPUT CONTROL OPTIONS
; Output frequency for coords (x), velocities (v) and forces (f)
nstxout = 0
nstvout = 0
nstfout = 0
; Output frequency for energies to log file and energy file
nstlog = 500
nstenergy = 500
; Output frequency and precision for xtc file
nstxtcout = 500
xtc-precision = 1000
; This selects the subset of atoms for the xtc file. You can
; select multiple groups. By default all atoms will be written.
xtc-grps = BNZ
; Selection of energy groups
energygrps = BNZ
; NEIGHBORSEARCHING PARAMETERS
; nblist update frequency
nstlist = 10
; ns algorithm (simple or grid)
nstype = grid
; Periodic boundary conditions: xyz, no, xy
pbc = xyz
periodic_molecules = no
; nblist cut-off
rlist = 1.5
; OPTIONS FOR ELECTROSTATICS AND VDW
; Method for doing electrostatics
coulombtype = Cut-off
rcoulomb-switch = 0
rcoulomb = 1.5
; Relative dielectric constant for the medium and the reaction field
epsilon_r = 1
epsilon_rf = 1
; Method for doing Van der Waals
vdw-type = Cut-off
; cut-off lengths
rvdw-switch = 0
rvdw = 1.5
; Apply long range dispersion corrections for Energy and Pressure
DispCorr = No
; Extension of the potential lookup tables beyond the cut-off
table-extension = 1
; Seperate tables between energy group pairs
energygrp_table =
; Spacing for the PME/PPPM FFT grid
fourierspacing = 0.12
; FFT grid size, when a value is 0 fourierspacing will be used
fourier_nx = 0
fourier_ny = 0
fourier_nz = 0
; EWALD/PME/PPPM parameters
pme_order = 4
ewald_rtol = 1e-05
ewald_geometry = 3d
epsilon_surface = 0
optimize_fft = no
; IMPLICIT SOLVENT ALGORITHM
implicit_solvent = No
; GENERALIZED BORN ELECTROSTATICS
; Algorithm for calculating Born radii
gb_algorithm = Still
; Frequency of calculating the Born radii inside rlist
nstgbradii = 1
; Cutoff for Born radii calculation; the contribution from atoms
; between rlist and rgbradii is updated every nstlist steps
rgbradii = 2
; Dielectric coefficient of the implicit solvent
gb_epsilon_solvent = 80
; Salt concentration in M for Generalized Born models
gb_saltconc = 0
; Scaling factors used in the OBC GB model. Default values are OBC(II)
gb_obc_alpha = 1
gb_obc_beta = 0.8
gb_obc_gamma = 4.85
; Surface tension (kJ/mol/nm^2) for the SA (nonpolar surface) part of GBSA
; The default value (2.092) corresponds to 0.005 kcal/mol/Angstrom^2.
sa_surface_tension = 2.092
; OPTIONS FOR WEAK COUPLING ALGORITHMS
; Temperature coupling
tcoupl = berendsen
; Groups to couple separately
tc-grps = BNZ
; Time constant (ps) and reference temperature (K)
tau_t = 0.1
ref_t = 300
; Pressure coupling
Pcoupl = Berendsen
Pcoupltype = Isotropic
; Time constant (ps), compressibility (1/bar) and reference P (bar)
tau-p = 1.0
compressibility = 4.5e-5
ref-p = 1.0
; Scaling of reference coordinates, No, All or COM
refcoord_scaling = No
; Random seed for Andersen thermostat
andersen_seed = 815131
; OPTIONS FOR QMMM calculations
QMMM = no
; Groups treated Quantum Mechanically
QMMM-grps =
; QM method
QMmethod =
; QMMM scheme
QMMMscheme = normal
; QM basisset
QMbasis =
; QM charge
QMcharge =
; QM multiplicity
QMmult =
; Surface Hopping
SH =
; CAS space options
CASorbitals =
CASelectrons =
SAon =
SAoff =
SAsteps =
; Scale factor for MM charges
MMChargeScaleFactor = 1
; Optimization of QM subsystem
bOPT =
bTS =
; SIMULATED ANNEALING
; Type of annealing for each temperature group (no/single/periodic)
annealing =
; Number of time points to use for specifying annealing in each group
annealing_npoints =
; List of times at the annealing points for each group
annealing_time =
; Temp. at each annealing point, for each group.
annealing_temp =
; GENERATE VELOCITIES FOR STARTUP RUN
gen-vel = no
gen-temp = 300
gen-seed = -1
; OPTIONS FOR BONDS
constraints = all-bonds
; Type of constraint algorithm
constraint-algorithm = Lincs
; Do not constrain the start configuration
continuation = no
; Use successive overrelaxation to reduce the number of shake iterations
Shake-SOR = no
; Relative tolerance of shake
shake-tol = 0.0001
; Highest order in the expansion of the constraint coupling matrix
lincs-order = 1
; Number of iterations in the final step of LINCS. 1 is fine for
; normal simulations, but use 2 to conserve energy in NVE runs.
; For energy minimization with constraints it should be 4 to 8.
lincs-iter = 1
; Lincs will write a warning to the stderr if in one step a bond
; rotates over more degrees than
lincs-warnangle = 30
; Convert harmonic bonds to morse potentials
morse = no
; ENERGY GROUP EXCLUSIONS
; Pairs of energy groups for which all non-bonded interactions are excluded
energygrp_excl =
; WALLS
; Number of walls, type, atom types, densities and box-z scale factor for Ewald
nwall = 0
wall_type = 9-3
wall_r_linpot = -1
wall_atomtype =
wall_density =
wall_ewald_zfac = 3
; COM PULLING
; Pull type: no, umbrella, constraint or constant_force
pull = no
; NMR refinement stuff
; Distance restraints type: No, Simple or Ensemble
disre = No
; Force weighting of pairs in one distance restraint: Conservative or Equal
disre-weighting = Conservative
; 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
nstdisreout = 100
; Orientation restraints: No or Yes
orire = no
; Orientation restraints force constant and tau for time averaging
orire-fc = 0
orire-tau = 0
orire-fitgrp =
; Output frequency for trace(SD) and S to energy file
nstorireout = 100
; Dihedral angle restraints: No or Yes
dihre = no
dihre_fc = 9999.0
; Free energy control stuff
free-energy = no
init-lambda = 0
delta-lambda = 0
sc-alpha = 0
sc-power = 0
sc-sigma = 0.3
couple-moltype =
couple-lambda0 = vdw-q
couple-lambda1 = vdw-q
couple-intramol = no
; Non-equilibrium MD stuff
acc-grps =
accelerate =
freezegrps =
freezedim =
cos-acceleration = 0
deform =
; Electric fields
; Format is number of terms (int) and for all terms an amplitude (real)
; and a phase angle (real)
E-x =
E-xt =
E-y =
E-yt =
E-z =
E-zt =
; User defined thingies
user1-grps =
user2-grps =
userint1 = 0
userint2 = 0
userint3 = 0
userint4 = 0
userreal1 = 0
userreal2 = 0
userreal3 = 0
userreal4 = 0
benzene.itp:
[ defaults ]
; nbfunc comb-rule gen-pairs fudgeLJ fudgeQQ
1 3 yes 0.5 0.5
[ atomtypes ]
; full atom descriptions are available in ffoplsaa.atp
; name bond_type mass charge ptype sigma epsilon
opls_145 CA 6 12.01100 -0.115 A 3.55000e-01 2.92880e-01
opls_146 HA 1 1.00800 0.115 A 2.42000e-01 1.25520e-01
[ bondtypes ]
; i j func b0 kb
CA HA 1 0.10800 307105.6 ; PHE, etc.
CA CA 1 0.14000 392459.2 ; TRP,TYR,PHE
[ angletypes ]
; i j k func th0 cth
CA CA HA 1 120.000 292.880 ;
CA CA CA 1 120.000 527.184 ; PHE(OL)
[ dihedraltypes ]
; i j k l func coefficients
; OPLS Fourier dihedraltypes translated to Gromacs Ryckaert-Bellemans form
; according to the formula in the Gromacs manual.
X CA CA X 3 30.33400 0.00000 -30.33400
0.00000 0.00000 0.00000 ; aromatic ring
[ dihedraltypes ]
; Improper OPLS dihedrals to keep groups planar.
; (OPLS doesnt use impropers for chiral atoms).
; Since these functions are periodic of the form 1-cos(2*x), the are actually
; implemented as proper dihedrals [1+cos(2*x+180)] for the moment,
; to keep things compatible.
; The defines are used in ffoplsaa.rtp or directly in your .top ile.
#define improper_Z_CA_X_Y 180.0 4.60240 2
[ moleculetype ]
; Name nrexcl
BNZ 3
[ atoms ]
; nr type resnr residue atom cgnr charge mass
typeB chargeB massB
1 opls_145 1 BNZ CA1 1 -0.115 12.011 ; -0.1150
2 opls_145 1 BNZ CA2 2 -0.115 12.011 ; -0.2300
3 opls_145 1 BNZ CA3 3 -0.115 12.011 ; -0.3450
4 opls_145 1 BNZ CA4 4 -0.115 12.011 ; -0.4600
5 opls_145 1 BNZ CA5 5 -0.115 12.011 ; -0.5750
6 opls_145 1 BNZ CA6 6 -0.115 12.011 ; -0.6900
7 opls_146 1 BNZ HA7 1 0.115 1.008 ; -0.5750
8 opls_146 1 BNZ HA8 2 0.115 1.008 ; -0.4600
9 opls_146 1 BNZ HA9 3 0.115 1.008 ; -0.3450
10 opls_146 1 BNZ HA10 4 0.115 1.008 ; -0.2300
11 opls_146 1 BNZ HA11 5 0.115 1.008 ; -0.1150
12 opls_146 1 BNZ HA12 6 0.115 1.008 ; -0.0000
[ bonds ]
; ai aj funct c0 c1 c2 c3
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 ]
; ai aj ak funct c0 c1 c2 c3
2 1 6 1
2 1 7 1
6 1 7 1
1 2 3 1
1 2 8 1
3 2 8 1
2 3 4 1
2 3 9 1
4 3 9 1
3 4 5 1
3 4 10 1
5 4 10 1
4 5 6 1
4 5 11 1
6 5 11 1
1 6 5 1
1 6 12 1
5 6 12 1
[ dihedrals ]
; ai aj ak al funct c0 c1
c2 c3 c4 c5
6 1 2 3 3
6 1 2 8 3
7 1 2 3 3
7 1 2 8 3
2 1 6 5 3
2 1 6 12 3
7 1 6 5 3
7 1 6 12 3
1 2 3 4 3
1 2 3 9 3
8 2 3 4 3
8 2 3 9 3
2 3 4 5 3
2 3 4 10 3
9 3 4 5 3
9 3 4 10 3
3 4 5 6 3
3 4 5 11 3
10 4 5 6 3
10 4 5 11 3
4 5 6 1 3
4 5 6 12 3
11 5 6 1 3
11 5 6 12 3
[ dihedrals ]
; ai aj ak al funct c0 c1
c2 c3
2 6 1 7 1 improper_Z_CA_X_Y
6 2 1 7 1 improper_Z_CA_X_Y
1 3 2 8 1 improper_Z_CA_X_Y
3 1 2 8 1 improper_Z_CA_X_Y
2 4 3 9 1 improper_Z_CA_X_Y
4 2 3 9 1 improper_Z_CA_X_Y
3 5 4 10 1 improper_Z_CA_X_Y
5 3 4 10 1 improper_Z_CA_X_Y
4 6 5 11 1 improper_Z_CA_X_Y
6 4 5 11 1 improper_Z_CA_X_Y
1 5 6 12 1 improper_Z_CA_X_Y
5 1 6 12 1 improper_Z_CA_X_Y
[ pairs ]
6 3 1
6 8 1
7 3 1
7 8 1
2 5 1
2 12 1
7 5 1
7 12 1
1 4 1
1 9 1
8 4 1
8 9 1
2 5 1
2 10 1
9 5 1
9 10 1
3 6 1
3 11 1
10 6 1
10 11 1
4 1 1
4 12 1
11 1 1
11 12 1
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