  We present a new effective-one-body (EOB) Hamiltonian with next-to-leading
order (NLO) spin-spin coupling for black hole binaries endowed with arbitrarily
oriented spins. The Hamiltonian is based on the model for parallel spins and
equatorial orbits developed in [Physical Review D 90, 044018 (2014)], but
differs from it in several ways. In particular, the NLO spin-spin coupling is
not incorporated by a redefinition of the centrifugal radius $r_c$, but by
separately modifying certain sectors of the Hamiltonian, which are identified
according to their dependence on the momentum vector. The gauge-fixing
procedure we follow allows us to reduce the 25 different terms of the NLO
spin-spin Hamiltonian in Arnowitt-Deser-Misner coordinates to only 9 EOB terms.
This is an improvement with respect to the EOB model recently proposed in
[Physical Review D 91, 064011 (2015)], where 12 EOB terms were involved.
Another important advantage is the remarkably simple momentum structure of the
spin-spin terms in the effective Hamiltonian, which is simply quadratic up to
an overall square root. Moreover, a Damour-Jaranowski-Sch\"afer-type gauge
could be established, thus allowing one to concentrate, in the case of circular
and equatorial orbits, the whole spin-spin interaction in a single radial
potential.
