  We introduce an efficient lattice regularization scheme for quantum Monte
Carlo calculations of realistic electronic systems. The kinetic term is
discretized by a finite difference Laplacian with two mesh sizes, a and a',
where a'/a is an irrational number so that the electronic coordinates are not
defined on a particular lattice but on the continuous configuration space. The
regularized Hamiltonian goes to the continuous limit for a -> 0 and provides
several advantages. In particular, it allows the inclusion of non-local
potentials in a consistent variational scheme, substantially improving the
accuracy upon previous non-variational approaches.
