The atom charges for lipid differ between CHARMM-GUI and the CHARMM GROMACS port

User discussions
1

GROMACS version:2023.4
GROMACS modification: No
CHARMM-GUI assigns charges to certain atoms of POPC differently from those in the GROMACS CHARMM port (charmm36_ljpme-jul2022.ff). The specific atoms with differing charges are listed below. Which set of charges accurately represents POPC? It may be relevant to mention that the total charge (qtot) of the atoms listed below remains consistent for both sets.

Lipid.itp from GROMACS PORT
C2 CTL1 0.1339 3
HS HAL1 0.1023 3
O21 OSL -0.4739 3
C21 CL 0.8445 3
O22 OBL -0.6272 3
C22 CTL2 -0.1652 3
H2R HAL2 0.0928 3
H2S HAL2 0.0928 3
C3 CTL2 0.0302 4
HX HAL2 0.1030 4
HY HAL2 0.1030 4
O31 OSL -0.4739 4
C31 CL 0.8445 4
O32 OBL -0.6272 4
C32 CTL2 -0.1652 4
H2X HAL2 0.0928 4
H2Y HAL2 0.0928 4

CHARMM-GUI POPC.itp
CTL1 C2 0.170000
HAL1 HS 0.090000
OSL O21 -0.490000
CL C21 0.900000
OBL O22 -0.630000
CTL2 C22 -0.220000
HAL2 H2R 0.090000
HAL2 H2S 0.090000
CTL2 C3 0.080000
HAL2 HX 0.090000
HAL2 HY 0.090000
OSL O31 -0.490000
CL C31 0.900000
OBL O32 -0.630000
CTL2 C32 -0.220000
HAL2 H2X 0.090000
HAL2 H2Y 0.090000

2

There are two different models - one that if you intend to use LJ-PME for LJ interactions, and one for use with the normal force-switching scheme. The charges from CHARMM-GUI are for use with the switching scheme.

3

Thanks a lot for your reply. I will read more about the topic to understand it better.