Abstract
<p>The floating orbital molecular dynamics approach treats the basis functions' centers in ab initio molecular dynamics simulations variationally optimized in space rather than keeping them strictly fixed on nuclear positions. An implementation of the restricted theory for closed shell systems is already available (Perlt et al., Phys. Chem. Chem. Phys., 2014, 16, 6997–7005). In this article, the extension of the methodology to the unrestricted theory in order to cover open shell systems is introduced. The methyl radical serves as a test system to prove the correctness of the implementation and to demonstrate the scope of this method. The available spin density plots and vibrational spectra are compared to those obtained from atom-centered bases. Finally, more complex systems as well as further properties to be studied in future investigations by floating orbitals are suggested.</p>
Highlights
In recent years, the investigation of more and more complex systems by multi-scalar methods became feasible (Masson, Laino, Röthlisberger, & Hutter, 2009) (Ihrig, Schiffmann, & Sebastiani, 2011) (Kurzbach, Sharma, Sebastiani, Klinkhammer, & Hinderberger, 2011) (Golze, Iannuzzi, Nguyen, Passerone, & Hutter, 2013)
The floating orbital molecular dynamics approach treats the basis functions' centers in ab initio molecular dynamics simulations variationally optimized in space rather than keeping them strictly fixed on nuclear positions
The investigation of more and more complex systems by multi-scalar methods became feasible (Masson, Laino, Röthlisberger, & Hutter, 2009) (Ihrig, Schiffmann, & Sebastiani, 2011) (Kurzbach, Sharma, Sebastiani, Klinkhammer, & Hinderberger, 2011) (Golze, Iannuzzi, Nguyen, Passerone, & Hutter, 2013). This progress was only possible by introducing alternatives to standard methods. An example for such an alternative is the usage of floating orbitals which means that the centers of the basis functions for the construction of the wave function are not necessarily located on the nuclear positions but are optimized in space in order to minimize the total energy
Summary
The investigation of more and more complex systems by multi-scalar methods became feasible (Masson, Laino, Röthlisberger, & Hutter, 2009) (Ihrig, Schiffmann, & Sebastiani, 2011) (Kurzbach, Sharma, Sebastiani, Klinkhammer, & Hinderberger, 2011) (Golze, Iannuzzi, Nguyen, Passerone, & Hutter, 2013). The general idea is to distinguish between centers of basis functions ρ and nuclear coordinates A when formulating the molecular integrals, which arise when applying the Hamiltonian to basis functions for the construction of the self-consistent field (SCF) equations This is in contrast to conventional methods, where basis functions are centered on their respective nuclei. In molecular dynamics (MD) simulations the unwanted Pulay forces arise due to incomplete local basis sets and can be eliminated by the usage of floating orbitals (Marx & Hutter, 2009). In this contribution, the extension of the floating orbital molecular dynamics approach to unrestricted Hartree–Fock theory is presented. The paper closes with a summary and a brief outlook on future studies
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