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Generalized hybrid-orbital method for combining density functional theory with molecular mechanicals
Jingzhi Pu1, Jiali Gao, Donald G Truhlar
1Department of Chemistry and Supercomputing Institute, University of Minnesota, 207 Pleasant Street S.E, Minneapolis, MN 55455-0431, USA.
Summary
The generalized hybrid orbital (GHO) method is extended to density functional theory (DFT) and hybrid DFT (HDFT) for accurate QM/MM calculations. This new GHO-(H)DFT approach efficiently includes electron-correlation effects, improving molecular mechanics simulations.
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Molecular Modeling
Background:
- The Generalized Hybrid Orbital (GHO) method previously combined molecular mechanics with quantum mechanics (QM) at various levels (semiempirical, ab initio Hartree-Fock, DFT-tight binding).
- Accurate and efficient inclusion of electron-correlation effects in QM/MM calculations remains a challenge.
- Existing GHO formulations did not fully capture electron-correlation effects crucial for accurate molecular simulations.
Purpose of the Study:
- To extend the Generalized Hybrid Orbital (GHO) method to Density Functional Theory (DFT) and Hybrid Density Functional Theory (HDFT).
- To enable accurate and efficient incorporation of electron-correlation effects within QM/MM calculations.
- To develop analytical gradients for the new GHO-(H)DFT formalism.
Main Methods:
- The GHO method was extended to generalized gradient approximation (GGA) DFT and hybrid DFT (HDFT) using Gaussian-type orbitals.
- Auxiliary hybrid orbitals were used to compute total electron density, satisfying orthonormality via Löwdin orthogonalization.
- Analytical gradients were formulated, incorporating forces from GHO basis transformations, and scaling parameters were optimized.
Main Results:
- The GHO-(H)DFT method was tested using GGA (BLYP, mPWPW91) and HDFT (B3LYP, mPW1PW91, MPW1K) functionals.
- Evaluations included geometries and atomic charges for small molecules, C-C stretch potential in ethane, and torsional barriers in n-butane.
- Proton affinities, alanine dipeptide conformational energies, and hydrogen-atom transfer barriers were accurately reproduced.
Conclusions:
- The developed GHO-(H)DFT method provides an accurate and efficient approach for QM/MM calculations, effectively handling electron-correlation.
- The method demonstrates good performance in predicting molecular geometries, atomic charges, and various energetic properties.
- GHO-(H)DFT offers a robust framework for complex molecular systems where QM/MM boundary effects are significant.