@article{oai:nagoya.repo.nii.ac.jp:00007066, author = {Yasuda, Koji}, issue = {5}, journal = {PHYSICAL REVIEW A}, month = {May}, note = {In this paper two fundamental questions in the contracted Schrödinger equation (CSE) approach are considered by using Lipkin’s quasispin model: 1-1 mapping between the second-order reduced density matrix (2-RDM) and the wave function of an excited state, and the uniqueness of the solution of CSE under incomplete N-representability conditions. We present some examples of the wave functions that give the same 2-RDM as the excited state. Thus 2-RDM of an excited state does not determine the wave function uniquely, and it alone cannot be used as basic variable for excited states of the density-matrix theory. Under the incomplete representability constraints the solution of the second-order CSE contains all the exact 4-RDMs together with the spurious ones. We examined the distribution of the solutions as a function of energy, and found that the solutions are well separated from each other under the P- and G-representability conditions of 4-RDM in the low-energy region, but with moderate interaction, or in the higher-energy region, there exist spurious solutions for almost all energies. Thus the G condition of 4-RDM is not sufficient to solve the excited states, although it gives accurate results for the ground state of Lipkin’s model.}, pages = {52121--11}, title = {Uniqueness of the solution of the contracted Schrödinger equation}, volume = {65}, year = {2002} }