# Surprising Performance for Vibrational Frequencies of the Distinguishable Clusters with Singles and Doubles (DCSD) and MP2.5 Approximations

## TL;DR

This paper demonstrates that the DCSD method can accurately compute vibrational spectra at near-CCSD(T) quality with lower computational cost, and MP2.5 offers a viable alternative for certain systems, outperforming some double hybrids without empirical parameters.

## Contribution

The study shows that DCSD achieves near-CCSD(T) accuracy for vibrational spectra at CCSD cost, and MP2.5 is effective for single-reference systems, providing a cost-effective alternative.

## Key findings

- DCSD yields vibrational spectra comparable to CCSD(T).
- MP2.5 performs well for vibrational frequencies in single-reference systems.
- DCSD is suitable for zero-point vibrational energy calculations.

## Abstract

We show that the DCSD (distinguishable clusters with all singles and doubles) correlation method permits the calculation of vibrational spectra at near-CCSD(T) quality but at no more than CCSD cost, and with comparatively inexpensive analytical gradients. For systems dominated by a single reference configuration, even MP2.5 is a viable alternative, at MP3 cost. MP2.5 performance for vibrational frequencies is comparable to double hybrids such as DSD-PBEP86-D3BJ, but without resorting to empirical parameters. DCSD is also quite suitable for computing zero-point vibrational energies in computational thermochemistry.

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Source: https://tomesphere.com/paper/1705.01889