UT Chem/Biochem Dept

The 2008-2009 Beckman Scholars: Devin A. Matthews

Beckman research project in the Stanton group:

An extension of the equation-of-motion coupled cluster method to biradicals

The equation-of-motion coupled cluster (EOM-CC) method,^1,2 is a powerful theoretical tool for the treatment of molecules with unpaired electrons, known as radicals. This approach, which exploits the structure of Fock space (the space spanned by all SCF determinants for all numbers of electrons), is based on the calculation of a "well-behaved" N-electron (closed shell) reference state. This information is then used to affect a similarity transformation of the electronic Hamiltonian and diagonalize the transformed Hamiltonian. When the Hamiltonian is diagonalized in the N-electron space, the method gives the excitation spectrum of the molecule of interest. Diagonalization in other sectors of Fock space (a different number of electrons) gives energies of specific target states. For instance, diagonalization in the (N-1)-electron space gives doublet radicals (one unpaired spin). The advantages of these approaches include rigorous spin-adaptation, balance in the treatment of different electron configurations, and ease of application.

The usual (N-1)-electron version of EOM-CC is to be extended to the singles, doubles and triples truncation of the CC equations (EOM-CCSDT). This will allow for a more accurate treatment of doublet radicals than is currently available with EOM-CCSD.² Perhaps more importantly, this method is also to be extended to utilize (N-2)-electron target states which will provide a useful technique for the study of so-called biradicals, which are ubiquitous in the chemistry of reactive intermediates found in processes such as combustion and the chemistries of the atmosphere and interstellar space.^3-6