Spin-orbital representation, with first-order

At this point, a comment on terminology is in order. In Chapter 1, we used the term ON vector for second-quantization vectors that correspond to Slater determinants in first quantization. The unusual term ON vector was employed in order to make a clear distinction between the first-and second-quantization representations of the same object and to emphasize the separate and independent structure of second quantization. We shall from now on use the conventional term Slater determinant or simply determinant for ON vectors, bearing in mind that determinants in second quantization are just vectors with elements representing spin-orbital occupations. 

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