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Quantum mechanic is superior to molecular mechanic in case of :• Modeling system with metal atoms• High accuracy• Analyze reaction mechanism / covalent bond formation or breaking• Prediction spectroscopy’s spectra that involve electrons (example : UV spectra)• Prediction of HOMO-LUMO energyQuantum Mechanic (QM)The correct mathematical description of the behavior of electronsin atoms and molecules.The core of QM method Schrödinger equationThe QM methods are oriented toward the molecular orbital (MO) approach.Schrödinger Equationis a wave equationinvolving the kinetic energy of the electrons and Coulombic interaction between electrons and nuclei.The kinetic energy of the nuclei is treated separately Born-Oppenheimer approximation>Simplified the Schrödinger equation without losing the accuracyGeometry Optimization :• Search the solution of Schrödinger equation Fact :Only Schrödinger equation for 1 hydrogen was solved exactlyATOMIC ORBITALS –2 :Heisenberg’s uncertainty principle don’t know exact position and energy of electron > Density plotSemiempiric Method :• Semi empirical method quantum mechanic method• replace the most time consuming parts of those calculations with values parameterized from experimental results.• Short running time compare to other quantum mechanic methods• Loss some accuracyAb Initio Method :• Ab initiomethod quantum mechanic method• does not rely on calibration against measuredchemical parameters• From the first principle• (ab initio calculations give results in terms of fundamental physical constants –Planck’s constant, the speed of light, the charge of the electron)• The simplest kind of ab initiocalculation is a Hartree-Fockcalculation
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