# Dihedral Parameterization

From: Venky (venky.agas_at_gmail.com)
Date: Mon Aug 20 2007 - 19:17:28 CDT

Hi,

I am trying to parameterize a small molecule using ab-initio calculations
and I am new to parameterization. I have a few questions regarding Dihedral
angle parameter determination.
I optimized the geometry of my molecule and used the lowest energy
configuration to generate Potential energy surface scan of a particular
dihedral angle rotation (rigid PES scan without optimization at each
rotation) and below are the energies obtained from the calculation.

*******************************************
Summary of the potential surface scan:

N t16(deg) SCF
---- --------- -----------
1 -179.9018 -1782.33483
2 -167.9018 -1782.33287
3 -155.9018 -1782.32715
4 -143.9018 -1782.31720
5 -131.9018 -1782.30308
6 -119.9018 -1782.28451
7 -107.9018 -1782.26171
8 -95.9018 -1782.23616
9 -83.9018 -1782.21000
10 -71.9018 -1782.18683
11 -59.9018 -1782.16332
12 -47.9018 -1782.11096
13 -35.9018 -1781.95668
14 -23.9018 -1781.55212
15 -11.9018 -1780.77154
16 0.0982 -1780.21021
17 12.0982 -1780.77625
18 24.0982 -1781.55508
19 36.0982 -1781.95792
20 48.0982 -1782.11159
21 60.0982 -1782.16350
22 72.0982 -1782.18703
23 84.0982 -1782.21022
24 96.0982 -1782.23629
25 108.0982 -1782.26192
26 120.0982 -1782.28451
27 132.0982 -1782.30301
28 144.0982 -1782.31721
29 156.0982 -1782.32717
30 168.0982 -1782.33291
31 180.0982 -1782.33483
---- --------- -----------

The graph is a single spike at 0 degrees with two minima at 180 and -180. I
tried to fit the data using the dihedral parameter equation...
V=Kchi(1+cos((n*chi)-d)) but the plot doesn't look like a cosine curve.
rather the energy rise is steep at its maximum (near 0 degrees) and the data
doesn't fit the equation.
I used n=1, d=0 and varied Kchi over chi= -180 to 180.

Is the procedure followed by me the right way to go about dihedral
parameterization? If then how do I go about fitting the PES energies to the
Dihedral equation.