  State-of-the-art multi-reference energy density functional calculations
require the computation of norm overlaps between different Bogoliubov
quasiparticle many-body states. It is only recently that the efficient and
unambiguous calculation of such norm kernels has become available under the
form of Pfaffians~[L. M. Robledo, Phys. Rev. C79, 021302 (2009)].
  The goals of this work is (i) to propose and implement an alternative to the
Pfaffian method to compute unambiguously the norm overlap between arbitrary
Bogoliubov quasiparticle states and (ii) to extend the first point to
explicitly correlated norm kernels at play in recently developped
particle-number-restored Bogoliubov coupled-cluster (PNR-BCC) and
particle-number-restored many-body perturbation (PNR-BMBPT) ab initio
theories~[T. Duguet and A. Signoracci, J. Phys. G44, 015103 (2017)]. Point (i)
constitutes the purpose of the present paper while point (ii) is addressed in a
forthcoming companion paper.
  We generalize the method used in~[T. Duguet and A. Signoracci, J. Phys. G44,
015103 (2017)] to obtain the norm overlap between arbitrary Bogoliubov product
states under a closed-form expression. The formula is physically intuitive,
accurate, versatile and relies on elementary linear algebra operations. It
equally applies to norm overlaps between Bogoliubov states of even or odd
number parity. Numerical applications illustrate these features and provide a
transparent representation of the content of the norm overlaps. Furthermore,
the closed-form expression extends naturally to correlated overlaps at play in
PNR-BCC and PNR-BMBPT. As such, the straight overlap between Bogoliubov states
is the zeroth-order reduction of more involved norm kernels to be studied in
the forthcoming paper.
