[gmx-users] tabulated potentials and soft core free energy - this should not work ....

[Michael Brunsteiner]

Hi,

i meant to perform a free energy calculation to get the chemical
potential of water, and made a number of input files based on the excellent tutorial
by Justin Lemkul.
(Without thinking) I also tried using a tabulated potential for electrostatics
with: coulombtype = User
but it seems to work - all the numbers in dhdl.xvg are finite and non-zero, and neither mdrun nor
grompp give me a warning) 
but then it shouldn't work ... how is Gromacs supposed to calculate the dU/dlambda term ?
or Is this calculated numerically?
or did i overlook something here?
below my mdp file

thanks for any advice!

michael



mdp:


integrator               = sd
nsteps                   = 100000
dt                       = 0.002
;
pbc                      = xyz
nstcomm                  = 100
comm_grps                = System
;
nstxtcout                =
0
nstenergy                = 100
nst_log                  = 0
nstxout                  = 0
nstvout                  = 0
;
rlist                    = 1.35
;
coulombtype              = User
rcoulomb                 = 1.2
vdw-type                 =
switch
rvdw-switch              = 1.0
rvdw                     = 1.2
;
tc_grps                  = system
tau_t                    = 1.0
ref_t                    = 300
;
Pcoupl                   = Berendsen
tau_p                    = 0.5
compressibility          =
4.5e-5
ref_p                    = 1.0
pcoupltype               = isotropic
;
constraints              = hbonds
lincs_iter               = 8
;
free_energy              = yes
init_lambda              = 0.50
delta_lambda             = 0
foreign_lambda           = 0.40 0.60
dhdl_derivatives         =
yes
sc_alpha                 = 0.5
sc_power                 = 1
sc_sigma                 = 0.5
nstdhdl                  = 100
couple-moltype           = w1
couple-lambda0           = vdw-q
couple-lambda1           = none
couple-intramol          = no



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