e storage required for a single determinant is, in principle, one bit per spinorbital, or 2 × N orb bits. However, because CPUs are designed to handle e ciently 64-bit integers, each spin part is stored as an array of 64-bit integers, the unused space being padded with zeros. e actual storage needed for a determinant is 2 × 64 × N int bits, where N int = (N orb -1)/64 + 1 is the number of 64-bit integers needed to store one spin part.