Home




We are a research group in theoretical chemical physics, particularly the area known as quantum chemistry and its application to problems in molecular spectroscopy.   Amongst the topics of research currently being pursued in our group are the following


Quantum Chemistry



Development, and extensions, of the equation-of-motion coupled-cluster method

Quasidiabatic approaches in coupled-cluster theory

Relativistic coupled-cluster theory

Improvement of algorithms for existing high-level quantum-chemical methods

Methods for calculating cross-sections for photoelectron spectra.

"Magic square" determining ideal strategy for permutations of four-index quantities (quadruple excitation amplitudes and increments to same) for use in solving the CCSDTQ equations. A very efficient implementation of CCSDTQ and CCSDT(Q) is a part of the new release of CFOUR.

Spectroscopy



Laboratory spectrum of Si3C, recorded at the Harvard-Smithsonian for Astrophysics by N.J. Reilly, D.L. Kokkin and M.C. McCarthy (black), together with simulation based on EOM-CCSDT calculations.    


The Si2C molecule was recently discovered to be abundant in the interstellar medium, the first molecule known in space to contain two copies of the very abundant silicon atom.    The discovery was built upon laboratory work of M.C. McCarthy and associates at Harvard University, which was done with the assistance of theoretical calculations done by our group.  The equilibrium structure of Si2C is shown in the figure as well as the family of molecules SixCy (x+y=3).   All of these molecules are of great astronomical interest, and have interesting electronic structures.   Si2C was the last of these to be characterized experimentally.


Energy level "spaghetti diagram" for the lithium trimer, a prototypical system that exhibits the Jahn-Teller effect.   The energy levels plotted here are obtained from a fourth-order (quartic) Hamiltonian, with the y-axis giving the level energy in cm-1 and the x axis a scaling parameter s, which scales the components of the Hamiltonian that are "anharmonic" (cubic and quartic in the diabatic picture). It is notable that the (second-order) quadratic model (s=0.0) gives the wrong ordering of the two lowest excited levels of the system, a somewhat surprising failure of the ubiquitous JT2 model for this "simple" system.  The levels in black have e' vibronic symmetry, while the red and green levels have aand asymmetries, respectively.

Development of model Hamiltonian approaches for the simulation, prediction and non-superficial understanding of spectra

Breakdowns of the Born-Oppenheimer model as manifested in molecular spectroscopy

Development of the xguinea and xsim spectral simulation packages, which are now a part of CFOUR


Chemical Kinetics


Above is a plot of the energy distribution in the so-called Criegee intermediate
(CH2COO), immediately after it is formed from the bimolecular reaction of ethylene and ozone.   The temperature and temperature of the reaction are taken to be 300 K and 1 atm, roughly simulating ambient atmospheric conditions, and <J> = 40.

The use and efficient implementation of semiclassical transition state theory (SCTST), which is a non-empirical theory that accounts for effects such as tunneling and path anharmonicity.



Master equation studies of reactions that occur in combustion or planetary atmospheres

Thermochemistry

Left graphic from Ruscic et al. J. Phys. Chem. A108, 9979 (2004)

Development of high-accuracy quantum-chemical methods for calculating bond energies, heats of formation, and related properites.


Mechanisms of Chemical Reactions


Schematic potential energy surface governing isomers of CH2O2 as well as the related dissociation products.   The celebrated Criegee intermediate is the least stable minimum energy structure on this potential surface, and is seen at the upper left.   



Energies (in kJ mol-1, relative to separated HO and CO)  of relevant points on the HOCO potential energy surface, with activation energies given in italics.   Apart from the pathway leading to the cis isomer of the title molecule, all values are HEAT-345(Q) calculations.    The formation of HOCO from HO+CO passes through a linear (OHCO)  pre-reactive complex which then leads to a trans transition state with the energy given in the lower right quadrant of the figure.   The energy given above for the {\it cis} pathway is that of the {\it trans} transition state plus the FC-CCSD(T)/ANO1 energy difference between it and the corresponding cis conformer, which is a second-order saddle point.

Kinetics and reaction mechanisms associated with the thermal decomposition of biomass, which are also studied by tunable (synchrotron) photoionization mass spectroscopy and infrared spectroscopy





 


We are now in the process of moving to the University of Florida, where John Stanton is currently located.   He is a member of the Quantum Theory Project as well as the William R. Kenan, Jr.  Professor of Chemistry.   


Two-dimensional mass spectrum of cyclohexanone, obtained by Jessie Porterfield, Oleg Kostko, 
Musa Ahmed and Barney Ellison using synchrotron radiation at the Advanced Light Source (Lawrence Berkeley National Laboratory) in the Autumn of 2014.   The two axes are m/z (x axis) and photon energy (y axis, in eV).   
The peak at m/z=98 is due to the parent species; the appearance of other masses at energies above 10.5 eV is due to dissociative ionization via cyclohexanone+ or its (more stable) enol+ isomer.


SOFTWARE



Together with the groups of J. Gauss (Mainz, Germany) and P.G. Szalay (Budapest, Hungary), we develop the CFOUR quantum chemistry package, which is freely distributed to all interested parties.   For more information about CFOUR, see: www.cfour.de

A new release of CFOUR should happen no later than autumn 2017


We are involved in the Scalable High-Performance Computing Group, which is headed by Prof. Robert van de Geijn in the computer science department at UT-Austin.    For more information, look here.

     

Another community effort of our research group is the MultiWell program suite for chemical kinetics calculations, a project that is headed by Prof. John Barker of the Department of Climate and Space Sciences and Engineering at the University of Michigan in the great city of Ann Arbor.    For more information,go here and have a read.   A chemical kinetics discussion group has also been formed, and can be found here.



We are privileged to be a part of the Active Thermochemical Tables project, a revolutionary advance in thermochemistry developed by Branko Ruscic at Argonne National Laboratory.   The development of ATcT has had a profound impact on thermochemistry; the enthalpies of formation for several key molecular species are now known much more precisely than they were a decade ago, with reduction of error bars typically by an order of magnitude.   By providing accurate and precise information about the thermodynamic stability of molecules, ATcT  will have profound impact on the fidelity of modeling studies.   The current ATcT team consists of Ruscic and his group at ANL, our team, and that of  Prof. G. Barney Ellison at the University of Colorado.