It is demonstrated that the Schrödinger operator in Ĥ | ψk >= Ek | ψk > can be associated with a covariance matrix whose eigenvalues are the squares of the spectrum σ(Ĥ + Iζ) where ζ is an arbitrarily chosen shift. An efficient method for extracting σ(Ĥ) components, in the vicinity of ζ, from a few specially selected eigenvectors of the inverse of the covariance matrix is derived. The method en...

Adiabatic representation Representation in which the electronic wave functions are calculated for fixed (i.e., nonmoving) nuclei. Avoided intersection Case in which two potential energy surfaces come together but do not intersect. Conical intersection Case in which two potential energy surfaces intersect such that their separation decreases to zero linearly in the relevant nuclear coordinates. ...

It is well known that positrons can live long enough to form complexes with atoms. The theoretical study of these complexes is important to interpret experimental data from both positron scattering by atoms and molecules and positron annihilation spectroscopy. There are typically two different kinds of positron complexes. In one of them a positron attaches to a neutral or ionic atom or molecule...

The Born-Oppenheimer separation of the Schroedinger equation allows the electronic and nuclear motions to be solved in three steps. 1) The solution of the electronic wave function at a discrete set of molecular conformations; 2) the fitting of this discrete set of energy values in order to construct an analytical approximation to the potential energy surface (PES) at all molecular conformations...

Laser pulse-induced photodissociation of molecules in rare-gas solids is investigated by representative quantum wavepackets or classical trajectories which are directed towards, or away from, cage exits, yielding dominant photodissociation into different neighbouring cages. The directionality is determined by a sequence of reflections inside the relief provided by the slopes of the potential en...

The potential energy surfaces of stacked structures consisting of adenine (A) and 2-aminopurine (2AP) have been investigated in the gas phase. Both faceto-back (the double-ring system of one base exactly on top of that of the other one) and face-to-face (one base flipped by 180 ) A/A, 2AP/2AP and A/2AP stacks were considered. Minima and transition states were optimised at the counterpoise-corre...

The product rotational alignment 〈P2(Ĵ′‚k̂)〉 ) -0.35 ( 0.04, -0.15 ( 0.02 for CaCl(B), CaCl(A) are obtained from Ca + NOCl chemiluminescent reactions, respectively. The experiments are carried out under single collision conditions in a beam-gas apparatus. Quasi-classical trajectory (QCT) calculations for Ca + NOCl reaction have been carried out. The calculated results agree well with the experim...

The isotope effect in the scattering of methane is studied by wavepacket simulations of oriented CH4 and CD4 molecules from a flat surface including all nine internal vibrations. At a translational energy up to 96 kJ/mol we find that the scattering is still predominantly elastic, but less so for CD4. Energy distribution analysis of the kinetic energy per mode and the potential energy surface te...

We calculate the potential energy surface, the low-frequency vibrational modes, and the electronic structure of a ~5,5!@~10,10! double-wall carbon nanotube. We find that the weak interwall interaction and changing symmetry cause four pseudogaps to open and close periodically near the Fermi level during the soft librational motion at n&30 cm. This electron-libration coupling, absent in solids co...

Using the ab initio potential of Shin ef al. (to be published), we have calculated the bound states and infrared absorption spectrum of the van der Waals complex Ar.+CO. The results show that Ar.*CO cannot be treated as a quasirigid rotor, nor as a molecule with a free internal rotor. In particular, a transition to the first excited van der Waals bending level is predicted to be present in the ...