PACS 63.22.+m, 65.40.Ba Molecular chains with rotational degree of freedom in the plane of an adsorbing surface and quadrupolar interaction between linear molecules are investigated theoretically. It is found that alternation ordering of the molecules is preferable. Equations of rotational movement are derived and solved for linear oscillations. It is shown that dispersion relation has two optic branches of rotational excitations. The normal coordinates are found to be strongly dependent on dispersion. In the long wave limit normal coordinates are symmetric and antisymmetric. The heat capacity of the molecular chain is found at low temperature.

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Rotational excitations of molecular adsorbed layers are studied theoretically. Nonlinear dynamical equations are obtained with accounting of quadrupolar interactions between molecules and freezing of translational degrees of freedom. The equilibrium positions of the molecules are found to be experimentally observed structures with alternating rotational ordering of planar rotors along the direction to the nearest neighbor (for linear or square structures) under low temperature. Dynamical analysis gives an integral of motion (energy) of the chain that in the long-wave limit leads consequently to the existence of four phases. The first one corresponds to oscillations near equilibrium ordered states. The second phase corresponds to low-energy rotational excitations along 'valleys' (easy directions in the effective potential) that do not destroy strong correlations between molecules while structural data can show rotational disorder (melting). The third phase corresponds to an energy that is enough to travel between 'valleys'; only some 'islands' in the angle space are forbidden. Complete destruction of correlation when the energy is over the peaks of the effective potential corresponds to the fourth phase. Therefore rotational melting is a complex phenomenon that has several stages.

The 180 • twin boundary (stacking fault) is investigated in the hexagonal close-packed (hcp) lattice. It is shown that the interatomic interaction between neighbors within the boundary lowers symmetry as compared with that in hcp phase. An initial spherical form (hcp phase) is elongated along the shift direction of the atomic planes inside the boundary. We find the wave functions of the helium atom for (i) the spherical oscillator (within the hcp phase) and (ii) an anisotropic one (inside the boundary). To estimate the parameters of the thermodynamic potential we apply the model of spheres and ellipsoids for the isosurfaces of the probability. Method of the successive approximations is applied for the self-consistent description of the distorted wave functions of atoms. It is shown that the quantum behavior of the boundary atoms causes the following effects: (i) increasing of degree of overlap of the atomic wave functions within the twin boundaries, (ii) increasing of the quantum diffusion inside the twin boundaries, (iii) decreasing of energy and broadening of the boundary in comparison with the classical case.