The problem actually is, that you mixed up a dummy atom with a virtual site, I guess.

Virtual sites are not allowed to have a mass, as the error message claims. Also, a proper hydrogen should have a mass.

If you want a hydrogen to appear, you have to place a particle in A-state which has no non-bonded interaction, i.e. LJ & QQ interactions should be zero. If you want to prevent it from flying away, you should put bonded terms to the particle. In the B-state, the QQ and LJ should be there (whereas the LJ interaction for a hydrogen is 0 anyway).

21 opls_999 1 LG4 H21 21 0.0000 1.00800 opls_172 0.4650 1.00800

opls_999   DUM   0     1.008000     0.000    A     0.000000   0.000000

So, I expect, this is, how it should look like.
Be aware of the missing bonded terms in the force field for a DUM particle. Maybe, you want to call it H instead...

Regards

Maik Goette, Dipl. Biol.
Max Planck Institute for Biophysical Chemistry
Theoretical & computational biophysics department
Am Fassberg 11
37077 Goettingen
Germany
Tel.  : ++49 551 201 2310
Fax   : ++49 551 201 2302
Email : mgoette[at]mpi-bpc.mpg.de
        mgoette2[at]gwdg.de
WWW   : http://www.mpibpc.gwdg.de/groups/grubmueller/


Wang Qin wrote:
Hi there,
       I met the problem when I ran "grompp", the errors are:
ERROR 1 [file "po4lig4tip3.top", line 26741]:
  virtual site H21 (Res LG4-173) has non-zero mass 1.008
       Check your topology.
ERROR 2 [file "po4lig4tip3.top", line 26741]:
  virtual site H22 (Res LG4-173) has non-zero mass 1.008
       Check your topology.

Then I checked my topology file, here were the definition of LG4-173 H21 and H22: ( I want to convert the dummy atoms to H atoms) ;nr type resnr residue atom cgnr charge mass type_B charge_B mass_B 21 opls_999 1 LG4 H21 21 0.0000 1.00800 opls_172 0.4650 1.00800 22 opls_999 1 LG4 H22 22 0.0000 1.00800 opls_172 0.4650 1.00800

in which opls_999 was from what I defined by myself in the force field itp file:
opls_999   DUM   0     1.008000     0.000    V     0.000000   0.000000

I changed the both of the mass_A to 0.00000, but that didn't help. Does anyone have ideas on that?
       Thank you very much.

Regards,
Qin


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