Computational quantum chemistry underwent a qualitative change in the past 50 years. Fueled by the synergy between improvements in computer technology and algorithmic advances, it became a practical tool for the whole chemical community. My research has concentrated on ground or low excited state molecular systems. This field can be classified into six categories: (1) Moderately accurate calculations of the structures and properties of drug-sized main-group molecules (2) High accuracy calculations on small and medium sized systems (3) Calculations on molecules with heavy elements where relativity is important (4) Quantum treatment of very large non-periodic systems (5) Molecules with strong (nondynamical) electron correlation, including reactive systems (6) Free energy calculations on thermodynamic systems (with a large number of degrees of freedom): molecules in solutions, embedded in proteins, or large clusters Item (1) is a solved problem if the present DFT accuracy is sufficient. (2) and (3), and perhaps (4) will yield naturally to improved algorithms and computers. The talk will focus on categories (4)-(6). The Coulomb term in large systems can be treated very efficiently for pure DFT by an intermediate plane-wave expansion. We need to reduce the overhead from other computational steps and to increase the accuracy of non-hybrid DFT, or find fast algorithms for the exchange. Parallelization of the code is essential for large systems. An important challenge for strong correlation is to make the programs easy to use by non-specialists. Results in this area will be presented. Another goal is to reduce the exponential scaling of complete active space calculations without affecting accuracy. Efficient treatment of dynamical correlation and the inclusion of its coupling to nondynamical correlation are not yet fully solved problems. For thermodynamic systems, routine fully quantum free energy calculations will not be possible in the foreseeable future. The two main lines of research are: (A) Fitting force fields or semiempirical models to quantum data (B) Combined Quantum/Molecular Mechanics (QM/MM) or other embedding methods which treat the active region by quantum mechanics and the surroundings/solvent at a lower level, often by a force field. QM/MM allows the treatment of more difficult systems than force fields but is limited by the speed of the quantum calculations. Methods which speed up QM/MM calculations by a large margin will be discussed.

Award Address (ACS Award in Theoretical Chemistry sponsored by the ACS Division of Physical Chemistry). Molecular properties from quantum mechanical calculations 1967 - 2017: Synergy, successes, and challenges

253rd National Meeting (2017)

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