Up to 80 parameters in the Jastrow factor were optimized for every structure using the method of variance minimization described in chapter 5. In excess of 75% of the DMC correlation energy was obtained in all cases. No attempt to obtain ``benchmark'' accuracy VMC results was made: the aim was to obtain wavefunctions of sufficient accuracy for use in DMC calculations.

For the fixed-node DMC calculations, a timestep of 0.003 a.u. and an ensemble of 640 walkers were used. The DMC calculations averaged moves in length.

A special consideration was made for the
ring. The
ring is ``cumulenic'' whereas the other rings in the
study are ``polyacetylenic''^{8.2}and have two unique bond lengths. MCHF and CI calculations show that
the
, cumulenic rings have
significant multi-determinantal character, unlike the
rings. [161] Due to the likely
improvement in the trial wavefunction, a CISD calculation was
performed and the configurations with largest weight used to form a
multi-determinant trial wavefunction. 43 determinants, including the
HF reference determinant were selected. Although only a small
reduction in the variance of the trial wavefunction resulted (by a.u. to a.u.), a small reduction in DMC energy was
found due to the improved nodal structure of the wavefunction. Results
comparing single and multi-determinant wavefunctions for this isomer
are presented in the next section.