Subscripts index” definition

In order to refer to the p-order correlation matrices with p > 2, the notation needs to become more precise. Thus a left superscript has to be introduced. As can be seen from the definition, Eq. (91), the first index of this left superscript denotes the global order of the matrix, and the following indices denote the order of the different TRDMs involved. The right subscripts, denoting the element labels, coincide with those of the 2-RDM from which they derive. 

Here, the general index j [i = 1,..., M M = n 2N + l) ]oftheCOshas been replaced by double indexing (for future convenience), where h denotes the band indices and p the levels inside the bands. The subscript 2 at the bracket indicates that integration must be performed over the coordinates of the second electron while Pn is the exchange operator. The detailed form of the relativistic Hartree-Fock equation for a crystal involves the definitions of the Dirac matrices in the form... 

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