CI works by expanding the wavefunction of a system as a linear combination of multiple configurations. Each configuration is a Slater determinant, which is a mathematical expression of the antisymmetrized product of single-particle wavefunctions (spin-orbitals). The coefficients in this linear combination are determined by solving the Schrödinger equation, often using matrix diagonalization techniques. The inclusion of multiple configurations allows CI to capture electron correlation effects that are missed by single-configuration methods.
