Piotr Piecuch

Research

(Research Description PDF)

My research program focuses on (i) ab initio quantum theory of molecular electronic structure and other many-body systems, (ii) molecular properties, spectroscopy, and photochemistry, (iii) reaction mechanisms and dynamics, and (iv) theory of intermolecular forces. We design and apply quantum-mechanical methods that enable precise determination of potential energy surfaces and property functions for both existing and hypothetical molecular species in their ground and excited states. We are also interested in accurate quantum calculations for strongly correlated systems, weakly interacting molecular clusters, and atomic nuclei.

Quantum theory of molecular electronic structure. The key to understanding molecular electronic structure and dynamical behavior of molecules is an accurate assessment of the many-electron correlation effects. Our group focuses on the development and applications of new quantum-mechanical methods that include correlation, particularly on the coupled-cluster theory and its renormalized, active-space, extended, multi-reference, and response variants that allow us to study bond breaking, electronically excited states, electron-transfer processes, molecular properties in vibrationally and electronically excited states, and transition probability coefficients for various types of spectroscopy. We also develop approximate coupled-pair approaches for strongly correlated systems and local correlation coupled-cluster methods character­ized by the linear scaling of the CPU time with the system size and natural parallelism, and their multi-level extensions that can be applied to high accuracy ab initio calculations for systems with hundreds of atoms. Our primary interest is in high-accuracy methods that allow us to be predictive. We write computer codes for the standard and new coupled-cluster methods which are distributed world-wide through a popular electronic structure package GAMESS. Some of our methods are also available in NWChem and, in the original or modified form, Q-Chem and MRCC packages.

Many-body methods of quantum mechanics and nuclear physics. Our new ab initio methods for many-electron systems can be applied to other many-fermion systems, including atomic nuclei. We performed several highly successful ab initio coupled-cluster calculations for 4He, 16O, and valence systems around 16O using modern nucleon-nucleon interactions. We also carried out unprecedented coupled-cluster calculations for 56Ni and its isotopes. We are looking for the alternative approaches to accurate calculations for many-fermion systems with pair-wise interactions, including the use of two-body cluster expansions to represent the virtually exact many-fermion states.

Molecular properties, spectroscopy, and photochemistry. We use linear-response coupled-cluster methods, along with other ab initio approaches, to calculate molecular multipole moments and (hyper)polarizabilities and the effect of nuclear motion on these properties. We use first-principles theories to obtain rovibrational and electronic spectra, including van der Waals precursors of photo-induced charge-transfer reactions. We have demonstrated that the lowest excited state of methylcobalamin should be interpreted as metal-to-ligand charge-transfer excitation and that azulene possesses the doubly excited state below the ionization threshold, which can drive multi-photon ionization experiments related to Rydberg fingerprint spectroscopy. We have provided de­finitive information about structural and spec­troscopic properties of several organic biradicals and small metal nanoparticles, including, for example, geometries of low-energy isomers of Au8 and the photoelectron spectra of Ag3— and Au3—.

Reaction mechanisms and dynamics. We performed successful computational studies for several important organic chemistry reactions, including the Cope rearrangement of 1,5‑hexadiene, cycloaddition of cyclopentyne to ethyl­ene, thermal stereomutations of cyclopropane, and isomerization of bicyclo­ [1.1.0] butane to buta-1,3-diene. We carried out unprecedented coupled-cluster calculations for CuO2 and Cu2O2 systems, relevant to oxygen activation by metalloenzymes, for photoisomerizations of acetylacetone, for diffusion of atomic oxygen on the silicon surface, for proton-transfer reactions between the dithiophosphinic acids and water molecules, for aerobic oxidation of methanol on gold nanoparticles, and for the Co-C bond dissociation in methylcobalamin, relevant to catalytic properties of B12. We also studied the photo-induced charge-transfer (“harpooning”) reactions between alkali and alkaline earth metal atoms and halides. In particular, we combined ab initio and dynamical approaches to characterize quasi-bound states of van der Waals molecules that are precursors of these reactions.

Intermolecular interactions. Intermolecular potentials are a necessary ingredient for the determination of the structure, stability, and dynamics of weakly bound clusters and condensed phases. We are interested in pair-wise non-additive interactions, which are important when three or more atoms or molecules interact, and study interactions in dimers.