A modified Ehrenfest formalism for efficient large-scale ab initio molecular dynamics

We present in detail the recently derived ab-initio molecular dynamics (AIMD) formalism [Phys. Rev. Lett. 101 096403 (2008)], which due to its numerical properties, is ideal for simulating the dynamics of systems containing thousands of atoms. A major drawback of traditional AIMD methods is the necessity to enforce the orthogonalization of the wave-functions, which can become the bottleneck for very large systems. Alternatively, one can handle the electron-ion dynamics within the Ehrenfest scheme where no explicit orthogonalization is necessary, however the time step is too small for practical applications. Here we preserve the desirable properties of Ehrenfest in a new scheme that allows for a considerable increase of the time step while keeping the system close to the Born-Oppenheimer surface. We show that the automatically enforced orthogonalization is of fundamental importance for large systems because not only it improves the scaling of the approach with the system size but it also allows for an additional very efficient parallelization level. In this work we provide the formal details of the new method, describe its implementation and present some applications to some test systems. Comparisons with the widely used Car-Parrinello molecular dynamics method are made, showing that the new approach is advantageous above a certain number of atoms in the system. The method is not tied to a particular wave-function representation, making it suitable for inclusion in any AIMD software package.Comment: 28 pages, 5 figures, published in a special issue of J. Chem. Theory Comp. in honour of John Perde

Shape and Temperature Dependence of Hot Carrier Relaxation Dynamics in Spherical and Elongated CdSe Quantum Dots

Time-domain nonadiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation dynamics in a CdSe spherical quantum dot (QD) and an elongated quantum dot (EQD) with the same diameter. The band gap is smaller, and the electron and hole states are denser in the EQD than in the QD. Also, the band gap shows a stronger negative temperature dependence in the EQD than in the QD. Higher frequency phonons are excited and scattered with electrons at higher temperatures for both QD and EQD. The electron-phonon coupling is generally stronger in the EQD than in the QD. The hot electron decay rates calculated from nonadiabatic molecular dynamics show a weaker temperature dependence than the T(-1) trend in both QD and EQD, which is attributed to the thermal expansion effect. Furthermore, the relaxation of hot electrons proceeds faster and shows stronger temperature dependence in the EQD than in the QD. Our work demonstrates that the shape of quantum dots has a strong impact on the electron decay dynamics