  In an attempt to bypass the sign problem in quantum Monte Carlo simulation of
electronic systems within the framework of fixed node approach, we derive the
exclusion principle "Two electrons can't be at the same external isopotential
surface simultaneously" using the first postulate of quantum mechanics. We
propose the exact Coulomb-Exchange nodal surface i.e. the exact boundary
condition to solve the non-relativistic Schrodinger equation for the
non-degenerate ground state of atoms and molecules. This boundary condition was
applied to compute the ground state energies of N, Ne, Li2, Be2, B2, C2, N2,
O2, F2, and H2O systems using diffusion Monte Carlo method. The ground state
energies thus obtained agree well with the exact estimate of non-relativistic
energies.
