We have performed molecular dynamics simulation investigations of the phase behavior of an ensemble of rigid molecules interacting via a soft-core repulsive potential. The system consisted of 600 bead-necklace molecules each composed of 11 interaction centers (beads). The system manifested two liquid crystalline phases, a nematic phase and a smectic A phase. Initial points on the isotropic-nematic and nematic-smectic A (P,T) phase coexistence curves were established through rigorous calculation of chemical potentials. The Gibbs-Duhem integration method was subsequently employed to trace the isotropic-nematic and nematic-smectic A coexistence curves over a wide range of pressure and temperature. This simple model was found to capture qualitatively many of the features of the phase behavior of real thermotropic liquid crystals. The isotropic-nematic transition was found to be weakly first order. The enthalpy, entropy, and density of transition for the isotropic-nematic increased with increasing temperature, while the temperature (pressure) range over which the nematic phase is stable increased with increasing pressure (temperature). The nematic-smectic A transition was also found to be weakly first order and was accompanied an increase in the orientational order parameter and a decrease in the thermal fluctuations of the orientational order parameter, indicating coupling between positional order and thermal fluctuations of orientational order. However, contrary to mean-field predictions and the observed behavior for some real thermotropic liquid crystalline materials, the enthalpy of the nematic-smectic A transition was found to increase with increasing temperature/pressure.