Kinetically constrained ring-polymer molecular dynamics for non-adiabatic chemical reactions.

We extend ring-polymer molecular dynamics (RPMD) to allow for the direct simulation of general, electronically non-adiabatic chemical processes. The kinetically constrained (KC) RPMD method uses the imaginary-time path-integral representation in the set of nuclear coordinates and electronic states to provide continuous equations of motion that describe the quantized, electronically non-adiabatic dynamics of the system. KC-RPMD preserves the favorable properties of the usual RPMD formulation in the position representation, including rigorous detailed balance, time-reversal symmetry, and invariance of reaction rate calculations to the choice of dividing surface.

Dissociative photoionization of glycerol and its dimer occurs predominantly via a ternary hydrogen-bridged ion-molecule complex.

The photoionization and dissociative photoionization of glycerol are studied experimentally and theoretically. Time-of-flight mass spectrometry combined with vacuum ultraviolet synchrotron radiation ranging from 8 to 15 eV is used to investigate the nature of the major fragments and their corresponding appearance energies. Deuterium (1,1,2,3,3-D5) and (13)C (2-(13)C) labeling is employed to narrow down the possible dissociation mechanisms leading to the major fragment ions (C3H(x)O2(+), C2H(x)O2(+), C2H(x)O(+), CH(x)O(+)).

Restricted active space spin-flip (RAS-SF) with arbitrary number of spin-flips.

The restricted active space spin-flip (RAS-SF) approach is a multistate, spin-complete, variational and size consistent method applicable to systems featuring electronic (near-)degeneracies. In contrast to CASSCF it does not involve orbital optimizations and so avoids issues such as root-flipping and state averaging. This also makes RAS-SF calculations roughly 100-1000 times faster.

Restricted active space spin-flip configuration interaction: theory and examples for multiple spin flips with odd numbers of electrons.

The restricted active space spin flip (RAS-SF) method is extended to allow ground and excited states of molecular radicals to be described at low cost (for small numbers of spin flips). RAS-SF allows for any number of spin flips and a flexible active space while maintaining pure spin eigenfunctions for all states by maintaining a spin complete set of determinants and using spin-restricted orbitals. The implementation supports both even and odd numbers of electrons, while use of resolution of the identity integrals and a shared memory parallel implementation allow for fast computation.

Mechanism for singlet fission in pentacene and tetracene: from single exciton to two triplets.

Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process has been known for decades, most descriptions have assumed the necessity of a charge-transfer intermediate. This ab initio study characterizes the low-lying excited states in acene molecular crystals in order to describe how SF occurs in a realistic crystal environment.

Theoretical study of substituted PBPB dimers: structural analysis, tetraradical character, and excited states.

The radicaloid nature of para and meta 1,3-diborata-2,4-diphosphoniocyclobutane-1,3-diyl doubly substituted benzene is assessed from several electronic structure perspectives. Orbital occupation numbers computed by perfect pairing (PP), complete active space SCF (CASSCF), and restricted active space double spin-flip (RAS-2SF) reveal the presence of less than one unpaired electron in the planar molecules. Thus, the surprising stability of the "para tetraradical" can be rationalized by its moderate extent of radical character.

Computational Studies on the Pt(II)-Catalyzed Cycloisomerization of 1,6-dienes into Bicyclopropanes: A Mechanistic Quandary Evaluated by DFT.

The mechanism of the (bis(phosphanylethyl)phosphane)Pt(2+) catalyzed cyclo-isomerization reaction of 7-methyl-octa-1,6-diene to form 1-isopropylbicyclo[3.1.0]hexane was studied using computational methods.