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

[Andreas]

The gd_?? macros are

#define gd_4    180.000       5.86          1
; N-CHn-CHn-OA (lipid)	1.4
#define gd_36     0.000       8.62          3
; N-CHn-CHn-OA (lipid)	2.1
#define gd_20     0.000       5.09          2
; O-P-O- (dna, lipids)	1.2
#define gd_27     0.000       3.19          3
; O-P-O- (dna, lipids)	0.8

The two repeating dihedral definitions are certainly in the .top file, because I 
  have copied the DPPC definition from ffG53a5.rtp into an .itp file as a 
starting point for developing a 'good' DPPC model consistent with the Gromos 
53a5 force field.

Many thanks for your comments
Andreas

David van der Spoel wrote:
> Mark Abraham wrote:
>>> 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.
>>
> 
> I'm not sure this is correct: check your top file. If you have two 
> dihedrals in the top file both are evaluated and the effective 
> interaction is the sum of these. This is a way to fake a more 
> complicated dihedral term.
> 
>> Mark
>>
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> 
>