Projection and No-Pair Hamiltonians

Although we have efficient and reliable (numerical) schemes at our disposal for the safe application of variational techniques to the many-electron Dirac-Coulomb Hamiltonian, the formal difficulties with this Hamiltonian continue to raise discussions in mathematical physics [231,232]. 

All quasi-relativistic Hamiltonians in the following may be thought of as being surrounded by proper projection operators onto electronic bound or 

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