A spanning tree cohomology theory for links

In their recent preprint, Baldwin, Ozsv'{a}th and Szab'{o} defined a twisted version (with coefficients in a Novikov ring) of a spectral sequence, previously defined by Ozsv'{a}th and Szab'{o}, from Khovanov homology to Heegaard-Floer homology of the branched double cover along a link. In their preprint, they give a combinatorial interpretation of the $E_3$-term of their spectral sequence. The main purpose of the present paper is to prove directly that this $E_3$-term is a link invariant. We also give some concrete examples of computation of this invariant.