17

Aligning a molecule to a particular frame of reference (e.g., with the z-axis along a particular bond) is part of Avogadro for this reason: In Avogadro 1.x, there's an align tool You click one atom that will be set to the origin You click another atom that will be projected along the x, y, or z-axis You click 'align' and the molecular coordinates will be ...


13

It comes down to the fact that HF and KS both are variational methods. This short article by Julien Toulouse gives a great description of ways to compute static/dynamic response properties. I'll just summarize the relevant portion. We can compute derivatives of the energy with respect to any variable $x$ as: $$\frac{dE}{dx}=\frac{\partial E}{\partial x}+\...


8

I assume you're referring to eq 51 of the Hirata-Head-Gordon-Bartlett paper. One should note that these are not two-electron integrals, since there is only one spatial position; these are rather weighted four-center one-electron integrals. As always, the problem when you have four indices is that there is a huge number of integrals that come out, and you ...


3

I would agree that the best approach is to align your molecule in a way where your desired field is along one of the Cartesian axes. However, you can specify the direction/magnitude of the applied field using Gaussian. While the typical way of specifying a field Field=M+/-N only allows you to specify a multipole M and a magnitude N*.0001 a.u., you can give ...


1

Options 2 and 3 appear to be the same: the response is almost always the response of the energy, since the wave function is determined by the energy principle. To clarify: in many methods (e.g. Hartree-Fock or CC) one computes the derivative of the energy functional with respect to the property (e.g. polarizability or NMR shielding constants); this turns out ...


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