  The quantum Monte Carlo (QMC) is one of the most promising many-body
electronic structure approaches. It employs stochastic techniques for solving
the stationary Schr\" odinger equation and for evaluation of expectation
values. The key advantage of QMC is its capability to use the explicitly
correlated wave functions, which allow the study of many-body effects beyond
the reach of mean-field methods. The most important limit on QMC accuracy is
the fixed-node approximation, which comes from necessity to circumvent the
fermion sign problem. The size of resulting fixed-node errors depends on the
quality of the nodes (the subset of position space where the wave function
vanishes) of a used wave function. In this dissertation, we analyze the nodal
properties of the existing fermionic wave functions and offer new types of
variational wave functions with improved nodal structure.
