In this talk,the N-body reduced density matrix(RDM) approach for nonorthogonal orbitals and their applications to ab initio classical valence bond(VB) methods are presented.In the nonorthogonal orbital based RDM approach,the Hamiltonian matrix elements between internally contracted VB wave functions are explicitly provided,and finally expressed in terms of tensor contractions of electronic integrals and N-body RDMs of the reference VB wave function.By using the tensor analysis formulism of the RDM-based approach,together with the concept of seniority number,a hierarchy of VB methods is proposed and efficiently implemented.Test calculations show that the novel developments are able to push the VB theory to its new limits.