Titill:  Calculation of quantum mechanical rate constants directly from ab initio atomic forces 
Höfundur:  
Leiðbeinandi:  Hannes Jónsson 
Útgáfa:  2007 
Tungumál:  Enska 
Háskóli/Stofnun:  University of Washington 
Efnisorð:  Efnafræði; Doktorsritgerðir 
URI:  https://hdl.handle.net/20.500.11815/717 
Útdráttur:Harmonic quantum transition state theory (HQTST), sometimes referred to as ’instanton
theory’ or ’ImF theory’, has been implemented in an efficient way and tested. HQTST is
analogous to the more familiar classical harmonic transition state theory (HTST), where
the rate is estimated from the energy difference between a reactant state minimum and
a first order saddle point on the potential energy surface ridge that separates reactants
and the products, along with a prefactor derived from harmonic expansion of the potential
around both the minimum and the saddle point. The method described here makes use of a
generalized minimum mode following method to locate saddle points on the effective quantum
mechanical energy surface for discretized Feynman path integrals (FPI). The overall
computational cost of estimating rate constants with this method is relatively low and it is
possible to use directly atomic forces obtained from first principle calculations. The method
is also well suited for systems containing many degrees of freedom, on the order of a few
hundred. Usually, a well converged results is achieved with 500  700 force calls per system
replica used to represent the FPI.
The method has been tested on several one and twodimensional systems where more
accurate (or even analytical) solutions for the rate constant can be obtained. Not only is
it found to robust and fast, but accurate as well, yielding results within a factor of 23
from the exact values, indicating that the approximations inherent in the procedure are well justified for chemically relevant systems. In addition, the method has been used for
calculating the rate of various transitions involving hydrogen atoms or molecules where
the atomic forces are derived from empirical, semiempirical or first principle calculations.
Calculations presented here include the rate of hydrogen abstraction from gas phase H3BN3,
hydrogen atom diffusion in Ta and Pd, adsorption/desorption of H2 onto/from Cu(100) and
Cu(110) surfaces and hydrogenation of N on Ru(0001) surface. Comparison is made with
either higher level theoretical calculations or experimental results when available, and the
agreement is found to be good in all instances.


Athugasemdir:A dissertation submitted in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
University of Washington
2007
