Electron-positron correlation effect

The QMC method is ideally suited for mixed systems because electron-positron correlation, which is difficult to treat with Cl methods, is automatically treated correctly. Systems of up to a bit more than ten leptons are routinely treated. Effective core potential methods can be used to extend the method to larger systems. Expectation values of local operators for the distribution k 2 are calculated by straightforward sampling procedures, but nonlocal operators, such as those for the annihilation rate, are problematic and are under active investigation [12],... 

In Ref. 122 we raised the question of whether one can use basis functions (49) in non-BO quantum mechanical calculations of molecular diatomic systems containing positrons and whether those functions are capable of providing a proper representation for the positron-nucleus and positron-electron correlation effects in a diatomic system. e+LiH was chosen as a target system. Along with e+LiH, we also performed the calculations of HPs and Li+ because the total energies of these systems are needed for determining the dissotiation energy. 

The procedure sketched above has recently been applied to the ground state of medium-heavy helium-like ions. We have evaluated the effect of QED combined with electron correlation, defined as the interaction with at least two Coulomb interactions. The QED part is here restricted to first order and consists of non-radiative effects (retardation of the electromagnetic interaction and effect of virtual electron-positron pairs) as well as radiative effects (electron self-energy, vacuum polarization and vertex correction). 

These traps, (Fig. 6) and similar effects in the motion of holes and other charges through polymers, would eventually be correlated also with such structural probes as positron lifetimes in macromolecular solids. Extensive recent studies of positron lifetime are based on positronium decay. In this, the lifetime of o-positronium (bound positron-electron pair with total spin one) is reduced from about 140 nanoseconds to a few nanoseconds by "pick-off annihilation" in which some unpaired electron spins in the medium cause conversion quenching of orthopositronium to para-positronium. The speed of the t2 effect is supposed, among other things, to represent by pick-off annihilation the presence of defects in the crystalline lattice. In any case, what amounts to empty space between molecules can then be occupied by orthopositronium.(14,15,16) It is now found in linear polyethylene, by T. T. Wang and his co-workers of Bell Laboratories(17) that there is marked shift in positron lifetimes over the temperature range of 80°K to 300°K. For... 

This basic quantity may be related to the true electron momentum distribution when the positron wave function is known through calculations. The many-body factor y f) is known to play some role but it has been established that the correlations have no effect on the position of the Fermi surface breaks (Majumdar 1965). Equation (1) shows that positron annihilation is well suited for Fermi surface studies as only ni(k) has breaks. This can be further seen if one expresses the positron and electron wave functions as Bloch wave functions. One obtains... 

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