[gmx-users] GROMOS angle type ga_13 or ga_15?

nikpap nikpap at ipta.demokritos.gr
Thu Sep 6 01:32:47 CEST 2012

GROMOS 54A7 angle types ga_13 and ga_15 seem to be both applicable to
(straight or branched) aliphatic compounds. The force constant is the same
but the equilibrium angle slightly differs from 109.5 to 111.0 degrees. I
suppose that this difference accounts for the steric hindrance that makes
the angles of an aliphatic C atom deviate from the ideal tetrahedral value
when it is bonded to different substituents. However, the GROMOS forcefield
or the related papers do not suggest in which cases each angle type should
be used. Any suggestions?

I have noticed that there is also some confusion in literature. Schuler et
al. [J. Comput. Chem. 22, 1205-1218 (2001)] used ga_13 for CHn-CH0-CHn and
CHn-CH1-CHn while they used ga_15 for CHn-CH2-CHn. Hoeltje et al. [Biochim.
Biophys. Acta 1511, 156-167 (2001), available from
http://lipidbook.bioch.ox.ac.uk ], in his popular model for cholesterol,
followed a different approach.

Thank you for any ideas!

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