Dear everyone,

with the aim to check the exact potential parameters used using the AMBER ff implemented in MS 5.5 to simulate (by MD with Discover module) organic systems, I found problems to understand the exact potential form used for the torsion in relation to many references found in literature (as Cornell et al. JACS 1995, 117, 5179-5197..).

in order to be clearer..

here is the torsion form implemented in amber.frc (used by Discover)

#torsion_3 amber

> E = SUM(n=1,3) { (Vn/m) * [ 1 + cos(n*Phi - Phi0(n)) ] }

> m = multiplicity or total number of torsions centered on the same bond.

!Ver  Ref     I     J     K     L           V1     Phi0        V2     Phi0        V3     Phi0

!----     ---    ----  ----   ----   ----         -------    ------      -------    ------        -------  ------

...    ..      ..  . .     ..   ..    ..    ..     ..          ..       ..        ..     ..         ..      ..

1.0   1     *     CT    CT    *          0.0000    0.0      0.0000    0.0      1.3000    0.0

that seems a bit different from the torsion form used for example in Cornell et al. JACS 1995, 117, 5179-5197:

E = SUM(n=1,3) { (Vn/2) * [ 1 + cos(n*Phi - Phi0(n)) ]

especially..in the  (Vn/m) component....where in second expression it seems that m = 2 and that the multiplicity is n = 1,3.

the problem is that or I couldn't found the specific AMBER reference on which is based the amber.frc or something is not so clear..

Could you help me??

thanks in advance...

G