Efficient High‐Level Quantum Chemical Exploration of Clathrate Hydrates via Fragmentation

We present fragmentation-based MP2 and CCSD(T)-level energetic calculations on naturally occurring clathrates encapsulating gas molecules CO <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> <mml:annotation>$$ {}_2 $$</mml:annotation></mml:semantics> </mml:math> , CH <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mrow></mml:mrow> <mml:mn>4</mml:mn></mml:msub> </mml:mrow> <mml:annotation>$$ {}_4 $$</mml:annotation></mml:semantics> </mml:math> , and H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> <mml:annotation>$$ {}_2 $$</mml:annotation></mml:semantics> </mml:math> S, within a 20-water cage. These calculations are followed by the computation and analysis of their vibrational infrared (IR) spectra. High-accuracy single-point energy evaluations at the MP2 and CCSD(T) levels using the aug-cc-pVNZ basis sets (N = T, Q, 5), involving <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>∼</mml:mo></mml:mrow> <mml:annotation>$$ \sim $$</mml:annotation></mml:semantics> </mml:math> 6200 basis functions are performed on a desktop workstation with 16 cores, demonstrating the practical feasibility of such demanding computations. To the best of our knowledge, this work reports, for the first time, the complete basis set (CBS) limit at the CCSD(T) level for a system of this size. Our efficient and scalable in-house developed fragment-based algorithms, REAlgo and CIC, enable high-level correlated calculations with large basis sets, opening new avenues for the exploration of intermolecular interactions in complex molecular clusters.