Anharmonic coupling potential energy surfaces

Later work evaluated the two-dimensional potential energy surface using various correlation treatments including many-body perturbation theory and coupled cluster techniques Evaluation of the vibrational spectrum was explicitly anharmonic in nature, mak-... 

Of course, this corresponds to an adiabatic potential-energy surface with two potential-energy minima separated by a well-defined barrier. Note that y A contains anharmonicities induced into Xr by D/A interactions as well as energy corrections that originate from D/A coupling. To some extent (i.e., as in first-order perturbation theory or in a Taylor s series expansion around the diabatic values of... 

Figure 15.1 Anharmonic coupling ofthe O-H stretching mode q and a low-frequency hydrogen bond (0...0) mode Q. (a) Potential energy diagram for the low-frequency mode in a single hydrogen bond. The potential energy surfaces as defined by the stretching mode and the quantum levels ofthe low-frequency mode are plotted for the Voh = 0 and 1 states as a function of the slow-mode coordinate Q.

The adiabatic potential energy surfaces in the vicinity of a conical intersection usually exhibit a rather complicated form, i.e. they are strongly anharmonic and may show double or multiple minima. In contrast to the adiabatic surfaces, the diabatic ones often show an astonishingly simple structure and may — for the region of interest — be well approximated by simple harmonic potentials. Combined with the fact that the diabatic coupling is well behaved, non-singular, and of potential type, this makes the diabatic representation extremely attractive. 

All investigations presented in Sects. 4-6 have employed Bom-Oppenheimer dynamics (BOMD). We apply no scaling factor of any kind to the vibrations extracted from the dynamics. The sampling of vibrational anharmonicities, i.e., potential energy surface, dipole anharmonicities, mode couplings, anharmonic... 

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