Apparent difficulties that prevent the definition of canonical conjugates for certain observables, e.g., the number operator, are eliminated by distinguishing between the Heisenberg and Weyl forms of the canonical commutation relations (CCR's). Examples are given for which the uncertainty principle does not follow from the CCR's. An operator F is constructed which is canonically conjugate, in the Heisenberg sense, to the number operator; and F is used to define a quantum time operator.

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The determination of formation and migration energies of point and clustered defects in Sic is of critical importance to a proper understanding of atomic phenomena in a wide range of applications. We present here calculations of formation and migration energies of a number of point and clustered defect configurations. A newly developed set of parameters for the modified embedded-atom method (MEAM) is presented. Detailed molecular dynamics calculations of defect energetics using three representative potentials. namely the Peanon potential. the Tersoff potential and the MEAM. have k e n performed. Results of the calculations are compared to first-principles calculations and to available experimental data. The results are analysed in terms of developing a consistent empirical interatomic potential and are used to discuss various atomic migration processes.

We present a few results on the spectral properties of a class of physically reasonable non-Hermitian Hamiltonians. These theorems relate the spectral properties of a non-self-adjoint operator (of the aforementioned class) in terms of that of a self-adjoint operator. These theorems can be specialized to yield conditions under which the perturbed eigenvalues (of the above class of operators) vary continuously from the eigenvalues of the unperturbed operators. If the Schrödinger equation has to be solved numerically, a knowledge of the spectral properties of the non-Hermitian Hamiltonian would insure when the eigensolutions exist.

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