[gmx-users] DPPC lipid model in Gromos 53a5

[Mark Abraham]

> Dear all,
>
> In the definition of the DPPC lipid molecule in ffG53a5.rtp I found, that
> some
> dihedral angles are defined twice:
>
>   [ dihedrals ]
> ;  ai    aj    ak    al   gromos type
>    C33     N   C32   C31     gd_29
>      N   C32   C31   O32     gd_4
>      N   C32   C31   O32     gd_36   <-
>    C32   C31   O32     P     gd_29
>    C31   O32     P   O31     gd_20
>    C31   O32     P   O31     gd_27   <-
>    O32     P   O31    C3     gd_20
>    O32     P   O31    C3     gd_27   <-
>
> Is this a mistake or on purpose ?
>
> Does the second entry overwrite the first entry, i.e. the values of the
> second
> entry are used ?

It depends on the functional form - you'll need to look up these gd_??
macros in the forcefield .itp file. A second function with the same
function type will over-write the first. I think it does so silently. A
different function type on the same atoms is accepted silently.

Mark