Fig. 3

First-principles calculation of the phonon dynamics for orthorhombic CH₃NH₃PbI₃. a Calculated zero-Kelvin phonon density of states (pDOS) and phonon scattering phase space. b Absolute variation in the N-H (red), C-H (cyan), and C-N (blue) bond lengths from ab initio molecular dynamics (AIMD) simulations. Data are averaged over the four CH₃NH₃⁺ cations in the orthorhombic unit cell. Initial imparted energy is 164 meV (per unit cell). c Variation in the C-N bond angle for different, indicated amounts of energy imparted into the unit cell. The purple curve is for a representative C-N bond; orange and brown curves are data averaged over all of the four CH₃NH₃⁺ cations in the orthorhombic unit cell. Data in (b, c) are offset for clarity. d Transient ΔOD spectra acquired at several different delay times; inset shows the overlapped transient spectra in arbitrary units by simple scaling. Fluence used was 0.45 mJ cm⁻²