[gmx-users] Potential issue using acpype to convert amber topology to gromacs

[Justin Lemkul]

On 2/14/18 7:14 AM, Easton J.W. wrote:
> Hi,
>
>
> I've used acpype to convert my prmtop and inpcrd files into gromacs format. I'm using the amber14sb forcefield.
>
>
> At the top of the top file generated, is an [atomtypes] section. Most of the atoms in this section were present in the atomtypes.atp and ffnonbonded.itp so I deleted them however there were two atomtypes A2C and A3C that were not, so I left them.
>
>
> Looking back at my files after running some simulations I have realised that these were meant to be the 2C and 3C atomtypes, which had been converted. I've also realised that they do not have any mass in the atomtypes section.
>
>
> ---
>
>
> [ atomtypes ]
> ;name   bond_type     mass     charge   ptype   sigma         epsilon       Amb
>   A2C      A2C         0.00000  0.00000   A     3.39967e-01   4.57730e-01 ; 1.91  0.1094
>   A3C      A3C         0.00000  0.00000   A     3.39967e-01   4.57730e-01 ; 1.91  0.1094
>
> [ moleculetype ]
> ;name            nrexcl
> Peptide           3
>
> [ atoms ]
> ;   nr  type  resi  res  atom  cgnr     charge      mass       ; qtot   bond_type
>       1    N     1   CYS     N    1    -0.415700     14.01000 ; qtot -0.416
>       2    H     1   CYS     H    2     0.271900      1.00800 ; qtot -0.144
>       3   CX     1   CYS    CA    3     0.021300     12.01000 ; qtot -0.123
>       4   H1     1   CYS    HA    4     0.112400      1.00800 ; qtot -0.010
>       5  A2C     1   CYS    CB    5    -0.123100     12.01000 ; qtot -0.133
>       6   H1     1   CYS   HB2    6     0.111200      1.00800 ; qtot -0.022
>       7   H1     1   CYS   HB3    7     0.111200      1.00800 ; qtot 0.089
>       8   SH     1   CYS    SG    8    -0.311900     32.06000 ; qtot -0.223
>       9   HS     1   CYS    HG    9     0.193300      1.00800 ; qtot -0.029
>      10    C     1   CYS     C   10     0.597301     12.01000 ; qtot 0.568
>      11    O     1   CYS     O   11    -0.567901     16.00000 ; qtot -0.000
>
> ---
>
> What will this mean for the simulations that I have run?

The mass and charge information will be taken from [atoms], which always 
over-writes [atomtypes] for those quantities. So there should be no 
physical problem, e.g. from a "massless" atom. You can verify the masses 
by using gmx dump on your .tpr file and checking that they're non-zero 
for those atoms, but grompp would have failed before that point if they 
were truly massless.

-Justin

-- 
==================================================

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

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