| 2:52:48 PM PST - Mon, Jan 16th 2012 |
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Per MP2 documentation:
If an MP2 energy gradient is computed, all contributions are available to form the MP2 linear-response density. This is the density that when contracted with any spin-free, one-electron operator yields the associated property defined as the derivative of the energy. Thus, the reported MP2 dipole moment is the derivative of the energy w.r.t. an external electric field and is not the expectation value of the operator over the wavefunction. It has been shown that evaluating the MP2 density through a derivative provides more accurate results, presumably because this matches the way experiments probe the electron density more closely[5][6][7][8].
Only dipole moments are printed by the MP2 gradient code, but natural orbitals are produced and stored in the permanent directory with a file extension of ".mp2nos". These may be fed into the property package to compute more general properties. Note that the MP2 linear response density matrix is not necessarily positive definite so it is not unusual to see a few small negative natural orbital occupation numbers. Significant negative occupation numbers have been argued to be a sign that the system might be near degenerate[9].
You can use the .mp2nos file as an input to the property calculations.
Bert
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