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WIREs Comput Mol Sci
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VeloxChem: A Python‐driven density‐functional theory program for spectroscopy simulations in high‐performance computing environments

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Abstract An open‐source program named VeloxChem has been developed for the calculation of electronic real and complex linear response functions at the levels of Hartree–Fock and Kohn–Sham density functional theories. With an object‐oriented program structure written in a Python/C++ layered fashion, VeloxChem enables time‐efficient prototyping of novel scientific approaches without sacrificing computational efficiency, so that molecular systems involving up to and beyond 500 second‐row atoms (or some 10,000 contracted and in part diffuse Gaussian basis functions) can be routinely addressed. In addition, VeloxChem is equipped with a polarizable embedding scheme for the treatment of the classical electrostatic interactions with an environment that in turn is modeled by atomic site charges and polarizabilities. The underlying hybrid message passing interface (MPI)/open multiprocessing (OpenMP) parallelization scheme makes VeloxChem suitable for execution in high‐performance computing cluster environments, showing even slightly beyond linear scaling for the Fock matrix construction with use of up to 16,384 central processing unit (CPU) cores. An efficient—with respect to convergence rate and overall computational cost—multifrequency/gradient complex linear response equation solver enables calculations not only of conventional spectra, such as visible/ultraviolet/X‐ray electronic absorption and circular dichroism spectra, but also time‐resolved linear response signals as due to ultra‐short weak laser pulses. VeloxChem distributed under the GNU Lesser General Public License version 2.1 (LGPLv2.1) license and made available for download from the homepage https://veloxchem.org. This article is categorized under: Software > Quantum Chemistry Electronic Structure Theory > Density Functional Theory Theoretical and Physical Chemistry > Spectroscopy
VeloxChem program workflow and module overview
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First‐order interaction of noradrenaline with a Gaussian‐shaped optical pulse (ω0 = 0.32 a.u.; Δt = 30 a.u.; E0 = 10−5 a.u.; t0 = 300 a.u.). (a) Section of real and imaginary parts of the frequency‐domain isotropic electric‐dipole polarizability, , truncated by the region of appreciable field spectral amplitude, Fω(ω). (b) Time‐domain representation of the electric field, F(t), and the resulting first‐order‐induced dipole moment, . Computational details are identical to those in Figure
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Absorption and ECD spectra for DAPI in vacuum and bound to DNA in aqueous solution. Results are obtained at the level of TDA/BHandHLYP/6‐31+G(p,d). Oscillator strengths and rotatory strengths (in units of 10−40 esu2cm2) are broadened by a Lorentzian line profile with a half‐width at half‐maximum (HWHM) of 0.124 eV to produce the displayed spectra
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ECD spectra for the series of helicenes built from 6 to 30 phenyl rings. Results refer to the calculation of rotatory strengths at the level of Hartree–Fock/def2‐SVPD for up to the 20 lowest excited states, depending on system size, as to include the first bright state. These rotatory strengths are broadened by a Lorentzian line profile with a half‐width at half‐maximum (HWHM) of 0.124 eV to produce the displayed ECD spectra. Structures are optimized with the density functional based tight binding DFTB approach
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Scaling with respect to number of cores for the optimization of the ground state wave function. Speedups for one complete SCF cycle (orange circle) and the isolated associated Fock matrix construction (blue square) are reported separately and with reference to the calculation using 32 cores. Results are obtained for a titanium oxide nanoparticle, Ti165O330, at the level of Hartree–Fock/def2‐SV(P), resulting in 8,580 contracted and 18,810 primitive basis functions
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Convergence characteristics of linear complex response equation solver. (a) Residuals of convergence (a.u., logarithmic scale) for the three electric‐dipole operators for a single resonant frequency, ω = 7.3749 eV; (b) same as (a) but with parallel handling of 200 optical frequencies and, in addition, a shown dimension of the reduced space; (c) resulting electronic circular dichroism spectrum with rotatory strengths depicted by bars and reported in units of 10−40 esu2cm2, and (d) spectrum with oscillator strengths depicted by bars. Results are obtained for noradrenaline at the level of Hartree–Fock/aug‐cc‐pVDZ with a damping term γ = 1,000 cm−1 and adopting a molecular structure optimized at the level of B3LYP/cc‐pVTZ
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Representative SCF convergence characteristics of the two‐level direct inversion of the iterative subspace (DIIS) scheme implemented in VeloxChem. Results are presented in atomic units and obtained at the Hartree–Fock/def2‐SVP level of theory for a three‐armed organic system with 834 atoms, resulting in 7,626 contracted and 12,366 primitive basis functions
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Speedup of the density functional theory (DFT)‐kernel integration with respect to number of cores and with reference to the 32‐core (1 node) case. Results are obtained for a zinc porphyrin derivative at the level of B3LYP/def2‐SVPD (without zinc f‐functions) and using the program default grid level 4 to spawn 1,111,721 grid points
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Single instruction multiple data (SIMD) vectorization for primitive integral blocks with different angular momenta (red circles) and the corresponding scalar computational costs (green squares) to be understood in a relative sense. Performance profiling results in the inset are obtained for the C60 fullerene at the level of Hartree–Fock/def2‐SVP and recorded with the performance profiler Intel VTune Amplifier running on a dual‐socket (20‐core) Intel Xeon node that implements AVX2
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Theoretical and Physical Chemistry > Spectroscopy
Electronic Structure Theory > Density Functional Theory
Software > Quantum Chemistry

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