r²SCAN-3c - A Low-Cost "Swiss-Army-Knife" Composite Density-Functional-Theory Method

r²SCAN-3c is a composite quantum chemical method that gives high accuracy at a much lower computational cost than comparable methods. At Rowan, we use r²SCAN-3c for all kinds of applications. Here's some more info about this method, and why we like it so much:

What Goes Into r²SCAN-3c

The underlying r²SCAN functional is a regularized version of the SCAN functional, which fulfills many of the theoretical constraints expected of the exact exchange–correlation functional but suffered from horrific numerical instability. Previous attempts to fix these numerical convergence problems had come at the expense of constraint adherence; r²SCAN managed to restore constraint adherence while increasing numerical stability to a manageable level.

The result of this effort is a functional that performs very well on a large number of diverse benchmarks. In the authors' words:

...we argue that r²SCAN is the first mGGA functional that truly climbs up to the third rung of the Jacobs ladder without significant side effects (e.g., numerical instabilities or an overfitting behavior that leads to a bad performance for the mindless benchmark set).

r²SCAN-3c combines this underlying density functional with a specially optimized basis set (mTZVPP), a dispersion correction, and a bespoke geometric counterpoise correction that reduces basis-set superposition error. These corrections make it possible to dramatically reduce the number of basis functions, thus accelerating the speed of calculations, without introducing the typical pathologies associated with small basis sets.

r²SCAN-3c Works Well On Everything

In their paper, the authors undertake an extensive set of benchmarks to try and understand the applicability of r²SCAN-3c to all sorts of problems. Among the benchmarks they study:

• Bond lengths and angles
• Gas-phase rotational constants
• Non-covalent interaction distance and energy
• Main-group thermochemistry
• Main-group barrier heights
• Interaction energies in large non-covalent systems
• Conformer energies
• Thermochemistry for organometallic systems
• Lattice energies and volumes for molecular crystals
• Surface adsorption

In virtually every case, the performance of r²SCAN-3c is comparable to much slower conventional DFT methods employing large basis sets. In the authors' words (emphasis added):

the new and thoroughly tested composite method r²SCAN-3c provides benchmark-accuracy for key properties at a fraction of the cost of previously required hybrid/QZ approaches and is more robust than any other method of comparable cost. This drastically shifts the aforementioned balance between the computational efficiency and accuracy, enabling much larger and/or more thorough screenings and property calculations. In fact, the robustness and broad applicability of r²SCAN-3c caused us to rethink the very structure of screening approaches.

These results match our experience. In a recent preprint, we recommend r²SCAN-3c as one of the best choices for affordable modeling of highly strained biomolecules.

Running r²SCAN-3c

Compared to traditional DFT methods with big basis sets, r²SCAN-3c offers comparable accuracy while running 2–3 orders of magnitude faster. r²SCAN-3c is robust and broadly applicable, and we use it all the time here at Rowan—r²SCAN-3c calculations are the backbone of our bond-dissociation-energy predictions, our hydrogen-bond-basicity workflow, and many other Rowan workflows run every day.

Create an account for free to start running r²SCAN-3c calculations today!