[gmx-users] Problem in energy calculation

[Justin Lemkul]

On 10/3/18 7:08 AM, Gonzalez Fernandez, Cristina wrote:
> Dear Gromacs users,
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> I want to calculate the van der waals and electrostatic interaction energy  between a lipid and water. For that purpose, and following the gromacs tutorials I have performed the following steps:
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>    *   I have include: energygrps = Other Water  in the .mdp file to be used in the energy minimization. I copy the .mdp file:
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> ; minim.mdp - used as input into grompp to generate em.tpr
> integrator    = cg        ; Algorithm (steep = steepest descent minimization)
> emtol        = 1000.0      ; Stop minimization when the maximum force < 1000.0 kJ/mol/nm
> emstep          = 0.01      ; Energy step size
> nsteps        = 50000          ; Maximum number of (minimization) steps to perform
> energygrps      = Other Water
> ; Parameters describing how to find the neighbors of each atom and how to calculate the interactions
> nstlist            = 1            ; Frequency to update the neighbor list and long range forces
> cutoff-scheme       = Verlet
> ns_type            = grid        ; Method to determine neighbor list (simple, grid)
> coulombtype        = PME        ; Treatment of long range electrostatic interactions
> rcoulomb        = 1.4        ; Short-range electrostatic cut-off
> rvdw            = 1.4        ; Short-range Van der Waals cut-off
> pbc            = xyz         ; Periodic Boundary Conditions (yes/no)
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>    *   From the output of the minimization (em.gro) I have grouped Other (lipid) and Water:
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> gmx make_ndx -f em.gro -o index.ndx
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>> 1 | 11
>> q

You don't need a merged group. You need two groups for which the 
interaction energy will be computed.

>             gmx grompp -f nvt.mdp -c em.gro -p topol.top -n index.ndx -o nvt.tpr
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>             nohhup gmx mdrun -deffnm nvt &
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>    *   Finally, I used gmx energy to determine the electrostatic and van der waals energies. However, when I select 42 (Coul-SR: Other-Water) and 43 (LJ-SR: Other-Water) the results for both energies are 0 KJ/mol.

If you're running on a GPU and a pre-2018 version, this is exactly what 
you should expect. energygrps should only be used as a post-processing 
analysis, on CPU hardware. They are unnecessary for the actual 
simulation and are not compatible with GPUs.

-Justin

-- 
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Justin A. Lemkul, Ph.D.
Assistant Professor
Virginia Tech Department of Biochemistry

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