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Dipole Moments of Selected Small Molecules- A Computational Study

Dipole Moments of Selected Small Molecules- A Computational Study . Antal Zoltan -PhD candidate 6304-Computational Chemistry March 2010. Outline. Introduction How do we calculate dipoles? Theories and basis sets Experimental geometries Optimized geometries HF vs. Electron Correlation

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Dipole Moments of Selected Small Molecules- A Computational Study

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  1. Dipole Moments of Selected Small Molecules- A Computational Study AntalZoltan-PhD candidate 6304-Computational Chemistry March 2010

  2. Outline • Introduction • How do we calculate dipoles? • Theories and basis sets • Experimental geometries • Optimized geometries • HF vs. Electron Correlation • The curious case of Carbon Monoxide • Conclusions

  3. Introduction • Dipole moments: magnetic, bond, nuclear, electric, etc. • Vector quantity - Polarity – Debye(not SI) • Convention: chemist’s vs. physicist's • Why are they important? • Charge distribution affects exterior potential – determines the Hamiltonian – determines the wave function • Dipole moments directly result from charge distributions • Good and simple way to test theories and basis sets

  4. How do we calculate dipoles? • Experimentally – microwave spectroscopy (provides some info on sign and direction) • Theoretically: • Direct calculation of expectation value of dipole moment operator – 0th order perturbation value • Direct evaluation of the full derivative expression (dE/dλ)λ=0 for CI-type wavefunctions • Nuclear/electronic contribution - geometry

  5. Theories and basis sets • HF, B3LYP, MP2, CCSD(T)-Golden Method • STO-3G, 3-21G, 6-31G(d), cc-PVTZ • Ascending experimental dipole moment values (D) and known experimental geometries • Calculations in increasing theory/basis set direction using Gaussian 03

  6. Experimental geometries • Results in agreement with other sources • Basis set performance almost independent of theory • Small basis sets perform bad – same tendencies with all theories • Larger basis sets perform well

  7. MAD=0.53 D Exp. CO = 0.122 D Exp=5.83 D HF = 4.85 D

  8. 3-21G with Experimental Geometries MAD=0.48D

  9. MAD=0.32D

  10. MAD=0.1D

  11. Optimized geometries • Geometries are extremely important –NH3 • Small basis sets fail • STO-3G – too pyramidal • 3-21G – too planar • HF - as the basis set gets larger – better results • Electron correlation important

  12. 3-21G – Optimized Geometries

  13. MAD=0.30D

  14. MAD=0.1D MAD=0.2D

  15. HF vs. Electron Correlation • HF performs good with large basis sets, but has difficulties with low range dipoles (0-5D) • Electron correlation: theories perform good only if the right amount of correlation is included in the wave function • CO –favorite candidate for evaluating the performance of various theoretical models

  16. The curious case of Carbon Monoxide • HF/large basis set good, but predicts the wrong sign - vector • Electron correlation – better if right amount of corr. is included Experimental Most of theory/basis B3LYP/cc-PVTZ (0.122 D) set comb. (0.125 D) • CCSD(T) – usually small errors, but needs the right basis set

  17. Conclusions • Small basis sets fail • Larger basis set perform better • Amount of correlation is important • For the system to be studied – homework must be done first • Basis set optimization for specific system

  18. Thank You !

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