above which a fermion could erroneously feel a potential from two other fermions simultaneously. The scattering length was chosen to be such that the superconducting coherence length was less than the simulation cell size and the effective range was fixed at zero.

Trial wavefunction {#subsec:trialwf}
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We follow after previous work  [@riospairingorb] and employ a Slater-Jastrow trial wavefunction of the form ${\Psi_\mathrm{T}}= \mathrm{e}^{-J}\det[\phi({\mathbf{s}}_{i,j})]$. The determinant ensures the correct fermionic spin-symmetry. The pairing orbital $\phi({\mathbf{s}}_{i,j})$ comprises plane wave and polynomial expansions in the fermion separations ${\mathbf{s}}_{i,j}$. The plane wave part of the pairing wavefunction equals the exact Hartree-Fock ground state wavefunction for the non-interacting fermion gas, and in the presence of attractive interactions, the polynomial component of the pairing orbital can shift the nodal surface to smoothly transform to a superconducting wavefunction. To capture additional fermion correlations, we include the Jastrow factor $J$ which is a function of all opposite-spin fermion separations comprising a short range isotropic $u$ term, anisotropic $p$ terms [@drumjas], and a $\nu$ term [@nujas] that reflects the simulation cell symmetry.

Monte Carlo and expectation values {#sec:vmcdmc}
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VMC is used to optimise the trial wavefunction by finding the minimum energy with respect to its variational parameters. The 