(5.1) | ||

The Fourier coefficients of the general angle terms are evaluated as a function of the

(5.2) | ||

(5.3) | ||

(5.4) |

The expressions in the engery term are:

- the numbers of the bond-, angle-, torsion-, inversion- and the non bonded-terms.
- forceconstants of the bond- and angle-terms.
- bond distance and
*natural*bond distance of the two atoms and . - angle and
*natural*angle for three atoms . - Fourier coefficients of the general angle terms.
- torsion angle and
*natural*torison angle of the atoms . - height of the torsion barrier.
- periodicity of the torsion potential.
- inversion- or out-of-plane-angle at atom .
- height of the inversion barrier.
- Fourier coefficients of the inversions terms.
- distance and
*natural*distance of two non bonded atoms and . - depth of the Lennard-Jones potential.
- partial charge of atoms and dielectric constant.

The relaxation procedure distinguishes between molecules wih more than
90 atoms and molecules with less atoms. For *small* molecules it
consists of a Newton step followed by a linesearch step. For
*big* molecules a quasi-Newton relaxation is done. The BFGS
update of the force-constant matric is done [36,37,30,38]. Pulay's DIIS
procedure is implemented for *big* molecule to accelarate the
optimization [39,29].

The coordinates for any single atom can be fixed by placing an 'f' in the third to eighth column of the chemical symbol/flag group. As an example, the following coordinates specify acetone with a fixed carbonyl group:

$coord 2.02693271108611 2.03672551266230 0.00000000000000 c 1.08247228252865 -0.68857387733323 0.00000000000000 c f 2.53154870318830 -2.48171472134488 0.00000000000000 o f -1.78063790034738 -1.04586399389434 0.00000000000000 c -2.64348282517094 -0.13141435997713 1.68855816889786 h -2.23779643042546 -3.09026673535431 0.00000000000000 h -2.64348282517094 -0.13141435997713 -1.68855816889786 h 1.31008893646566 3.07002878668872 1.68840815751978 h 1.31008893646566 3.07002878668872 -1.68840815751978 h 4.12184425921830 2.06288409251899 0.00000000000000 h $end