[gmx-users] grompp problem: ERROR 12 [file "test.pdb.top", line 97]: atom MW4 (Res T4P-10) has mass 0

[Joern Lenz]

dear gromacs users,

i have the following problem:

i did md simulations of a solvated system (amber tip4p water) containg protein 
and DNA using the amber ports.
analyzing the trajectory of my simulation i found ONE interesting system 
confugurations which i wrote into a pdb file using the "trjconv - dump" 
command. 
now i want to merge the chains of this pdb file  by using "pdb2gmx -merge" 
command in order to apply distance restraints between the DNA and the protein 
in an upcoming simulation. 
therefore i changed the water nomenclature as advised on the amber ports for 
gromacs url from SOL (with OW, HW2, HW3 and MW4) into T4P (with OW, HW2, HW3 
and MW4). pdb2gmx works fine after adding T4P into the ffamber03.hdb.
Unfortunately grompp complains and prints the following warning for each T4P 
residues (formerly SOL) to stdout:
grompp problem: ERROR 12 [file "test.pdb.top", line 97]:
atom MW4 (Res T4P-10) has mass 0
i added the following lines into the ffamber03_tip4p.itp but the error is 
still there. HAs anyone an idea of how to get rid of this ?
Thank you very much in advance.
greetings
joern

[ moleculetype ]
; molname	nrexcl
T4P		
#ifdef _FF_AMBER03
1      amber99_63	1	T4P	 OW			1	0            16.00000
2	amber99_62	1	T4P	HW2		1	0.52	1.00800
3	amber99_62	1	T4P	HW3		1	0.52	1.00800
4	MW		1	T4P	MW4		1      -1.04	0.00000
#endif
; 	at type	      res nr  res name	at name	      cg nr	charge	mass
[ settles ]
; i	funct	doh	dhh
1	1	0.09572	0.15139

[ dummies3 ]
; Dummy from			funct	a		b
4	1	2	3	1	0.128012065	0.128012065

[ exclusions ]
1	2	3	4
2	1	3	4
3	1	2	4
4	1	2	3