|Abstract:||This work is divided into two parts. In the first part, short-range structure of deuteron is studied using a nonrelativistic Hamiltonian. The equidensity surfaces for spin projection Ms=O distributions are found to be toroidal in shape, while those of Ms=±1 have dumbbell shapes at large density. The toroidal shapes indicate that the tensor correlations have near maximal strength at the interparticle distance r < 2 fm. They provide new insights and simple explanations of the structure and electromagnetic form factors of the deuteron. In the second part, relativistic effects are studied using a relativistic Hamiltonian defined as the sum of relativistic one-body kinetic energy, two- and three-body potentials and their
boost corrections. Variational Monte Carlo method is used to study two kinds of relativistic effects in the binding energy of 3H and 4He. The first is due to the nonlocalities in the relativistic kinetic energy and relativistic one-pion exchange potential (OPEP), and the second is from boost interaction. The OPEP contribution is reduced by rv 15% by the relativistic nonlocality, which may also have significant effects on pion exchange currents. However, almost all of this reduction is canceled by changes in the kinetic energy and other
interaction terms, and the total effect of the nonlocalities on the binding energy is very small. The boost interactions, on the other hand, give repulsive contributions of ~ 0.4 (1.9) MeV in 3H (4He) and account for ~ 1/3 of the phenomenological part of the three-nucleon interaction needed in the nonrelativistic Hamiltonians.