Moreover, in the 1980s a general expression for the macroscopic stress tensor was presented, which enables static DFT calculations of bulk systems under pressure. However, from a quantum mechanical point of view, it must be noted that none of the terms in the electronic Hamiltonian includes pressure as an explicit variable. This complicates the calculation of electronic energies and molecular properties using DFT and wavefunction-based simulation methods. Only within the past 15 years or so, specialized approaches have been developed that circumvent this problem and allow the simulation of pressure at the molecular level (Section 2.3). These approaches facilitate the comparison of high-pressure single molecule and bulk effects. Moreover, the full power of ab initio simulation methods for, e.g., bonding and interaction analyses, as well as calculations of a wealth of pressure-induced spectroscopic effects can be exploited.