

Hola Jaime,

Infortunadamente solo puedo (o creo poder) responder una de tus preguntas.  La 
matriz de las integrales bielectronicas necesita ser calculada porque gaussian 
efectua el CI excitando conjuntos de electrones, de manera que necesitan 
conocerse los orbitales a-priori, de hecho, lo que se optimiza luego son los 
coeficientes de la expansion no los orbitales mismos. 

Espero haberte servido,

Albeiro.

Unfortunately I can only (or I think I can) answer one of your questions.
The two-electron integrals matrix needs to be calculated because
Gaussian carries out the CI by exciting pairs of (groups) electrons, therefore, 
the orbitals are needed apriori. In fact, what is optimized then are 
the expansion coefficients not the orbitals themselves.

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