Quantum-Onsager SCRF

The Quantum-Onsager SCRF Method. In the dipole-in-a-sphere (or quantum-Onsager or Born-Kirkwood-Onsager) SCRF method, the molecular cavity is a sphere of radius a and the interaction between the molecular charge distribution and the reaction field is calculated by approximating the molecular charge distribution as an electric dipole located at the cavity center with electric dipole moment fi. In 1936, Onsager showed that the electric field in the cavity (the reaction field) produced by the polarization of the solvent by fi is (in atomic units)... 

In the quantum-Onsager SCRF method, an uncharged solute molecule with no permanent dipole moment is unaffected by the solvent. In reality, the quadrupole and higher moments of the solute will interact with the solvent to give a reaction field. 

The quantum Onsager model, which has also been termed the Self-Consistent Reaction Field (SCRF) method, is the simplest of the continuum models used in solvation studies. In this model, which dates from the work of Kirkwood[44] and Onsager[45] in the 1930s, the solvent is represented by a continuous rmifonn dielectric with a static dielectric constant, e, surrounding a solute in a spherical cavity[46] - [48]. 

In an SCRF quantum-Onsager calculation, one starts by using a method such as HF, DFT, MP2, or whatever, to calculate an electron probability density p r) for the isolated molecule, preferably at an optimized geometry. is calculated from (13.128) in a wave-function-based method or fi-om (15.119) in DFT] One then calculates the electric dipole moment in vacuum from (13.144) as = —f dt +... 

An improvement on a spherical molecular shape is an ellipsoidal molecular shape. Quantum-Onsager and multipole-expansion SCRF calculations using an ellipsoidal cavity give better results than spherical-cavity results, but the improvement is not great. 

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