The canonical front form Hamiltonian for non-Abelian SU(N) gauge theory in 3+1 dimensions is mapped non-perturbatively on an effective Hamiltonian which acts only in the Fock space of a quark and an antiquark. The approach is based on the novel method of iterated resolvents and on discretized light-cone quantization, driven to the continuum limit. It is free of the usual Tamm-Dancoff truncation...

We have employed the Douglas-Kroll-Hess approximation to derive the perturbative Hamiltonians involved in the calculation of NMR spin-spin couplings in molecules containing heavy elements. We have applied this two-component quasirelativistic approach using finite perturbation theory in combination with a generalized Kohn-Sham code that includes the spin-orbit interaction self-consistently and w...

The vacuum diagram is calculated at second order for theories with self-interacting massless fields in the framework of finite causal perturbation theory. It is pointed out that the infrared behaviour of the vacuum diagram leads to unstable Fock vacua for QCD or massless QED, but not for quantum gravity. Therefore a radical rearrangement of the physical system must take place for such theories....

We present a relation which connects the propagator in the radial (Fock-Schwinger) gauge with a gauge invariant Wilson loop. It is closely related to the well-known field strength formula and can be used to calculate the radial gauge propagator. The result is shown to diverge in four-dimensional space even for free fields, its singular nature is however naturally explained using the renormaliza...

The canonical front form Hamiltonian for non-Abelian SU(N) gauge theory in 3+1 dimensions is mapped non-perturbatively on an e ective Hamiltonian which acts only in the Fock space of a quark and an antiquark. The approach is based on the novel method of iterated resolvents and on discretized light-cone quantization, driven to the continuum limit. It is free of the usual Tamm-Danco truncations o...

We consider atoms with closed shells, i.e., the electron number N is 2, 8, 10, ..., and weak electron-electron interaction. Then there exists a unique solution γ of the Dirac-Fock equations [D (γ) g,α, γ] = 0 with the additional property that γ is the orthogonal projector onto the first N positive eigenvalues of the Dirac-Fock operator D (γ) g,α. Moreover, γ minimizes the energy of the relativi...

We studied the linear sigma model withO(N) symmetry at nonzero temperature in the framework of the CornwallJackiw-Tomboulis formalism [1, 2]. Extending the set of two-particle irreducible diagrams by adding sunset diagrams to the usual Hartree-Fock contributions, we derived a new approximation scheme which extends the standard Hartree-Fock approximation by the inclusion of nonzero decay widths....