Thursday, November 2, 2017

quantum chemistry - Hellmann-Feynman Forces with Hartree-Fock


The energy in the Hartree-Fock approximation is given as:


EHF=ψHF|ˆH|ψHF=i,jPi,jHcorei,j+12i,j,k,lPi,jPl,k(ij||kl)+VNN



The geometrical derivative of the Hartree-Fock energy can be shown to be [1]:


EHFXA=i,jPi,jHcorei,jXA+12i,j,k,lPi,jPl,k(ij||kl)XAi,jQi,jSi,jXA+VNNXA


The Hellmann-Feynman Theorem states:


dEdXA=ψ|dˆHdXA|ψ


The Hellmann-Feynman Theorem implies that the energy derivative only depends on the parts of the Hamiltonian that have a dependency on the derivative. This leads to the geometrical derivative being equal to:


dEdXA=ψ|dˆVNNdXA+dˆVNedXA|ψ=ψ|dˆVNNdXA|ψ+ψ|dˆVNedXA|ψ=ψ|ZABZB(XBXA)R3AB|ψ+ψ|ZAiXiXAr3iA|ψ


Now the Hellmann-Feynman theorem can be applied to find the derivative geometrical of the Hartree-Fock energy:


EHF,HellmannFeynmanXA=ψHF|ˆHXA|ψHF=i,jPi,jVNe,ijXA+VNNXA


I tried to implement equation (2) and equation (3), and used them to calculate, the force between H and Li for different distances, with cc-pVDZ and cc-pVTZ. and got the following:


enter image description here



Hartree-Fock refers to equation (2) and Hellmann-Feynman refers to equation (3). I have read that the Hellmann-Feynman theorem is only valid for exact solutions, but should become a better and better approximation, when going towards the Hartree-Fock limit. As can be seen in the picture, equation (3) performs worse with the larger basisset cc-pVTZ. This now leads me to my question, were am I wrong in my understanding of the application of the Hellmann-Feynman theorem to the Hartree-Fock approximation?


[1] Attila Szabo, Neil S. Ostlund; Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory; equation C. 12




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