## Publications

#### Publication list

A full list of publications is maintained at Google Scholar.

#### Posts on new publications

#### Older publications

Some key papers preceding my time at Penn State:

*Intrinsic atomic orbitals: An unbiased bridge between quantum theory and chemical concepts*, J. Chem. Theory Comput., 9 4834 (2013)

In quantum chemistry, researchers tend to view molecules as a large set of numbers and complex equations which need to be solved. This article introduces a simple and versatile approach for connecting this first-principles point of view with intuitive chemical concepts (e.g., partial charges, bond orbitals, oxidation states, etc.). With only three equations, one can, for example, calculate the first-principles equivalent of a molecule’s Lewis structure. The article reports tests of the approach regarding various empirical laws and facts, and finds it to be consistent. Additionally, this may well be the computationally cheapest and simplest way of making localized molecular orbitals.*Density matrix embedding: A strong-coupling quantum embedding theory*, J. Chem. Theory Comput., 9, 1428 (2013)

This describes a way of splitting complex quantum systems into fragments, and treating them effectively independently, coupled only over a common mean field. The significance of this is that the fragments can be much smaller than the full system, and this way become accessible to powerful electronic structure methods. Using DMET, we could nearly exactly dissociate hydrogen rings (model systems of strong correlation)—using 2×2 FCI embedded in Hartree-Fock(!). The approach may turn out to be a key in the treatment of large transition metal clusters and other strongly correlated systems in the future.*Determining the Numerical Stability of Quantum Chemistry Algorithms*, J. Chem. Theory Comput., 7, 2387 (2011)

How can one determine the effect of floating point rounding errors on the results of very large C++/Fortran programs? This article provides a simple and practical answer, involving minimal source code changes (rather, the compilation process is changed… introducing noise via random rounding into computations). Using this approach we found that (T)-triples of CCSD can be done in single precision without loss of accuracy, and we uncovered a numerical instability in a almost universally used technique in molecular integral evaluation.*A new internally contracted multi-reference configuration interaction method*, J. Chem. Phys. 135, 054101 (2011)

This describes our implementation of a new MRCI program, which can deal with much larger molecules than before. The main challenge is the complex wave function expansion: The unfactorized equations had a length of 180 pages, so that most implementation aspects had to be automated. The article describes our approach to the implementation, introduces some ideas in couplings coefficient evaluation (in particular, the use of hole- and mixed density matrices to simplify equations), and investigates the accuracy and efficiency of the CW contraction scheme.*Simplified CCSD(T)-F12 methods: Theory and Benchmarks*

J. Chem. Phys. 130, 054104 (2009).

The article describes our research which effectively turned CCSD-F12 into practical methods for real-word applications. It summarizes various benchmarks to find useful combinations of calculation parameters, hacks (e.g., scaled triples), and the various aspects of the background theory and implementation (some subtle but important).*Explicitly correlated RMP2 for high-spin open-shell reference states*, J. Chem. Phys. 128, 154103 (2008).

The article introduced the CABS-singles correction, which is now universally applied in F12 treatments, and it brought open-shell F12 methods to the technical state of the art. But it also contributed to the theoretical understanding of F12: In particular, it introduced

the external contraction viewpoint of the F12 cluster-operator[*],

which is a much cleaner way of deriving F12 equations than the then-prevalent

pair function viewpoint.