Reliable  nonadiabatic  dynamics requires  accurate solution of the coupled state electronic structure problem.  Often this electronic  structure data is determined, 'on the fly'  where the electronic Schr�dinger  equation is solved as requested  by the dynamics program.    Repeated invocation of the electronic Schr�dinger equation solver,  severely limits the accuracy of the electronic wave function that can be used.  An alternative approach is to obtain a representation of the coupled potential energy surfaces using a functional form.  We identify two classes of problems, (i) bound state problems where no chemical bonds are broken and a single set of internal coordinates suffices and (ii) dissociative problems where chemical bonds are made and broken and more general, overcomplete, coordinate sets are used.  We describe a constrained least squares fitting procedure which determines a quasi-diabatic representation of coupled adiabatic potential energy  surfaces and their interactions.  The representation is based on adiabatic energies, energy gradients and derivative couplings obtained from accurate multireference configuration interaction wave functions.   It does not require second derivatives of any quantities.  Using Lagrange multiplier constraints the locus of seams of conical intersections and their local topography are exactly reproduced.  Since derivative couplings are included in the construction process the representation is both quantifiably quasi-diabatic and optimally  diabatic in a least squares sense. Applications will be presented.

Representation of accurate coupled potential energy surfaces for nonadiabatic dynamics

243rd National Meeting (2012)

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