Add default template for repositories related to basis downfolding

templates/default_template.md

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+---
+title: "Basis Downfolding Template"
+description: "Template for repositories related to basis downfolding techniques as described in arXiv:1502.04698."
+---
+
+# Basis Downfolding
+
+[arXiv:1502.04698](https://arxiv.org/abs/1502.04698) describes a method for "folding" correlations from Hamiltonians in larger basis sets into Hamiltonians in smaller basis sets. Thus, this is a method for obtaining minimal basis sets with accuracy much higher than something like STO-6G. The technique essentially seeks an effective Hamiltonian which describes ground states with more accuracy than would be possible with the usual Galerkin formulation. This technique seems especially important for quantum computers since most experiments are limited to a very small numbers of qubits.
+
+## Overview
+
+- **Objective**: Implement and explore the basis downfolding method.
+- **Applications**: Quantum computing, small qubit experiments.
+- **Key Concepts**: Effective Hamiltonian, Galerkin formulation, minimal basis sets.
+