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Journal of Chemical Information and Computer Sciences

Role of Wave Functions' Nodal Surfaces in Interpretation of the Pauli Principle

Evgenii A. Smolenskii, Peter P. Aristov, Sergey Ya. Itshenko, Sergey A. Shpilkin, Sergey N. Maximoff

Published Jan 1, 1996• 10 citations
Publisher

Abstract

Analysis of the relationship between the structure of the symmetrical permutation group and the topological structure of the configuration space makes it possible to perform complete separation of spin and spatial variables for multielectron wave functions. This separation of variables allows one to explain the role of nodal surfaces in intepreting and understanding the physical sense of the Pauli principle for systems of fermions. The suggested hypothesis about the form of the nodal surfaces permits one to find these surfaces a priori for arbitrary multielectron systems. The use of nodal surfaces for constructing trial wave functions is demonstrated by the example of calculating the energy of the Li atom in the ground state.

DOI
10.1021/ci950086t

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Analysis of the relationship between the structure of the symmetrical permutation group and the topological structure of the configuration space makes it possible to perform complete separation of spin and spatial variables for multielectron wave functions. This separation of variables allows one to explain the role of nodal surfaces in intepreting and understanding the physical sense of the Pauli principle for systems of fermions. The suggested hypothesis about the form of the nodal surfaces permits one to find these surfaces a priori for arbitrary multielectron systems. The use of nodal surfaces for constructing trial wave functions is demonstrated by the example of calculating the energy of the Li atom in the ground state.
The Pauli exclusion principle is a key postulate of the quantum theory and informs much of what we know about matter. In terms of electronic structure, the lone, deceptively simple mathematical requirement is that the total wave function be antisymmetric with respect to the exchange of any two electrons. However, visualizing the effect on the electron distribution presents a formidable challenge to students because of the complexity and number of variables involved. Approaches to gaining insight developed here include a quantitative analysis of two-dimensional configuration spaces as well as a qualitative assessment of interference patterns. For 2p x 1 2p y 1 states as representative examples, the analysis reveals that the Pauli principle directs electrons with opposing spins to codistribute along axes passing through the nucleus in the plane of the occupied orbitals. On the other hand, electrons with the same spin are never simultaneously in the same place or on the same axis; instead, they preferentially distribute in perpendicular directions.
A fermion node is subset of fermionic configurations for which a real wave function vanishes due to the antisymmetry and the node divides the configurations space into compact nodal cells (domains). We analyze the properties of fermion nodes of fermionic ground state wave functions for a number of systems. For several models we demonstrate that noninteracting spin-polarized fermions in dimension two and higher have closed-shell ground state wave functions with the minimal two nodal cells for any system size and we formulate a theorem which sumarizes this result. The models include periodic fermion gas, fermions on the surface of a sphere, fermions in a box. We prove the same property for atomic states with up to 3d half-filled shells. Under rather general assumptions we then derive that the same is true for unpolarized systems with arbitrarily weak interactions using Bardeen-Cooper-Schrieffer (BCS) variational wave function. We further show that pair correlations included in the BCS wave function enable singlet pairs of particles to wind around the periodic box without crossing the node pointing towards the relationship of nodes to transport and many-body phases such as superconductivity. Finally, we point out that the arguments extend also to fermionic temperature dependent/imaginary-time density matrices. The results reveal fundamental properties of fermion nodal structures and provide new insights for accurate constructions of wave functions and density matrices in quantum and path integral Monte Carlo methods.
Enforcing the Pauli principle in many-body systems of fermions to ensure an antisymmetric wave function is typically a numerically expensive task. Numerical methods, such as coupled-cluster, correlated basis function, Monte Carlo, and traditional configuration interaction, scale with powers of NNN that make the determination of the energy spectrum of these systems a challenge. In a recent paper, we successfully obtained energies for large systems of fermions in the unitary regime in which we applied the Pauli principle ``on paper'' without obtaining an explicit wave function. This approach, an extension of the symmetry invariant perturbation method to fermions, applies the Pauli principle by imposing restrictions on the quantum numbers used to obtain the energy. As a followup to this study, we perform an explicit test of the validity of this application of the Pauli principle by comparing our energy results against the energies corresponding to explicitly antisymmetrized wave functions for an exactly solvable system of harmonically confined, harmonically interacting fermions. Our results show that our simple method of applying the Pauli principle selects from a spectrum of possible many-body energies only those energies that correspond to explicitly antisymmetrized wave functions. Our results were tested for values of NNN through $\ensuremath{\approx}10 000$ and for weak and strong interactions both repulsive and attractive.
The symmetrization postulate (SP) states that wave functions are either completely symmetric or completely antisymmetric under permutations of identical particles. It is shown by one-dimensional counter-examples that SP is not demanded by the usual physical interpretation of the mathematical formalism of wave mechanics unless one makes use of further physical properties of real systems; the error in a standard proof of SP which ignores these properties is pointed out. It is then proved that SP is true for those systems of spinless particles which have the following properties: (a) probability densities are permutation-invariant, (b) allowable wave functions are continuous with continuous gradient, (c) the 3n3n3n-dimensional configuration space is connected, (d) the Hamiltonian is symmetric, and (e) the nodes of allowed wave functions have certain properties. The counterexamples show that SP is not a necessary property of those systems which do not have property (c). The proof is extended to particles with internal degrees of freedom (including spin) by noting that any observable commutes with every permutation and making use of a particular observable acting only on internal variables. Extraneous mathematical assumptions, such as that of the existence of a "complete" set of commuting observables, criticized by Messiah and Greenberg, are not employed. Some comments are made on the conventional nature of the concept of identity for similar particles; the equivalence between the usual formulation in which different species of similar particles are treated as distinct, and that in which they are regarded as identical particles in different internal states, is emphasized.
There are many examples in theoretical physics where a fundamental quantity can be considered a quadratic form ρ=∑iρi=∣Ψ∣2 and the corresponding linear form Ψ=∑iψi is highly relevant for the physical problem under study. This, in particular, is the case of the density and the wave function in quantum mechanics. In the study of N-identical-fermion systems we have the additional feature that Ψ is a function of the 3N configuration space coordinates and ρ is defined in three-dimensional real space. For many-electron systems in the ground state the wave function and the Hamiltonian are to be expressed in terms of the configuration space (CS), a replica of real space for each electron. Here we present a geometric formulation of the CS, of the wave function, of the density, and of the Hamiltonian to compute the electronic structure of the system. Then, using the new geometric notation and the indistinguishability and equivalence of the electrons, we obtain an alternative computational method for the ground state of the system. We present the method and discuss its usefulness and relation to other approaches.