Explicitly correlated trial wavefunctions in quantum Monte Carlo calculations of excited states of Be and Be-

We present a new form of explicitly correlated wavefunction whose parameter s are mainly linear, to circumvent the problem of the optimization of a lar ge number of nonlinear parameters usually encountered with basis sets of ex plicitly correlated wavefunctions, With this trial wavefunction we have suc ceeded in minimizing the energy instead of the variance of the local energy , as is more common in quantum Monte Carlo methods. We have applied this wa vefunction to the calculation of the energies of Be P-3 (1s(2)2p(2)) and Be - S-4 degrees (1s(2)2p(3)) by variational and diffusion Monte Carlo methods . The results compare favourably with those obtained by different types of explicitly correlated trial wavefunction already described in the literatur e. The energies obtained are improved with respect to the best variational ones found in the literature, and within one standard deviation of the esti mated non-relativistic limits.