A combined experimental and computational approach is applied to amorphous polycarbonates (PCs) to link the macroscopic behavior and molecular features. The Simha-Somcynsky equation of phase (EOS) provides a more accurate description than the Tait EOS for describing the pressure-volume-temperature (PVT) relationship and thermal expansivity. The relationship between the specific volume and the hole free volume was investigated using PVT measurements, positron annihilation lifetime spectra, and molecular dynamics (MD) techniques, to examine the correlations between changes in the behavior of specific volume and molecular packing. We found that the specific volume and hole free volume have linear relationships with temperature. However, the occupied volume displayed remarkable compressibility. The analyses revealed that the PC with side methyl groups and long C-Si bonds contributed to the formation of large free volume. The MD results validate the effectiveness of molecular simulation techniques in designing materials at the molecular level.