Hessian matrices coupling coefficients

Since the energy is not a quadratic function of the coefficients, several Newton steps are needed to locate a minimum. In addition, the calculation and inversion of the full Hessian matrix can be extremely time consuming for large active spaces and/or basis sets. Often, the elements of the Hessian coupling orbital and configurational coefficients (cross derivatives) are neglected and the optimization of the two sets of coefficients are decoupled. 

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