fermions on a sphere surface, in a periodic box, atomic states, etc, and are based on algebraic arguments and BCS variational wavefunctions [1,2]. The correct nodal topology is of a key importance for quantum Monte Carlo fixed-node methods, and for this purpose we propose a new wavefunction which includes both singlet and triplet pairing functions inside a pfaffian . Using a testing set of first row atoms and molecules, we show that wavefunctions based on pfaffians are comparable to much larger configuration interaction expansions. For small systems we compare Hartree-Fock, pfaffian, and essentially exact wavefunctions and analyze the structure of corresponding nodal manifolds.
* in collaboration with L.K. Wagner, M. Bajdich, G. Drobny, K.E Schmidt (Arizona State U.).
 L. Mitas, PRL 96, 240402 (2006)
 L. Mitas, cond-mat/0605550.
 M. Bajdich, L.K. Wagner, G. Drobny, L. Mitas, K. E. Schmidt, PRL 96,130201 (2006)