Abstract
Linear molecules usually represent a special case in rotational-vibrational calculations due to a singularity of the kinetic energy operator that arises from the rotation about the a (the principal axis of least moment of inertia, becoming the molecular axis at the linear equilibrium geometry) being undefined. Assuming the standard ro-vibrational basis functions, in the 3N−6 approach, of the form ∣ν1,ν2,ν3ℓ3;J,k,m⟩, tackling the unique difficulties of linear molecules involves constraining the vibrational and rotational functions with k=ℓ3, which are the projections, in units of ℏ, of the corresponding angular momenta onto the molecular axis. These basis functions are assigned to irreducible representations (irreps) of the C2v(M) molecular symmetry group. This, in turn, necessitates purpose-built codes that specifically deal with linear molecules. In the present work, we describe an alternative scheme and introduce an (artificial) group that ensures that the condition ℓ3=k is automatically applied solely through symmetry group algebra. The advantage of such an approach is that the application of symmetry group algebra in ro-vibrational calculations is ubiquitous, and so this method can be used to enable ro-vibrational calculations of linear molecules in polyatomic codes with fairly minimal modifications. To this end, we construct a—formally infinite—artificial molecular symmetry group D∞h(AEM), which consists of one-dimensional (non-degenerate) irreducible representations and use it to classify vibrational and rotational basis functions according to ℓ and k. This extension to non-rigorous, artificial symmetry groups is based on cyclic groups of prime-order. Opposite to the usual scenario, where the form of symmetry adapted basis sets is dictated by the symmetry group the molecule belongs to, here the symmetry group D∞h(AEM) is built to satisfy properties for the convenience of the basis set construction and matrix elements calculations. We believe that the idea of purpose-built artificial symmetry groups can be useful in other applications.
Highlights
To describe the rotation and vibration of a well-bent triatomic molecule, such asH2 S, with a large, non-accessible barrier to linearity, a molecular kinetic energy operator (KEO) in terms of the 3 vibrational and three rotational coordinates [2] is constructed.The three vibrational modes correspond to the standard bent-molecule methodology that is based on the 3N − 6 vibrational modes
For the description of a linear triatomic molecule, for example CO2, whose potential energy surface would allow for the linear configuration to become feasible during vibration, we cannot proceed as described for H2 S above, as the corresponding KEO becomes singular at linear molecular geometries
We investigate whether the D∞h (M) symmetry description of the basis functions in Equation (3), naturally arising for the 3N − 6 case, can be extended in such a manner that it becomes similar to the much more detailed description, in terms of D∞h (EM), obtained for the 3N − 5 case
Summary
To describe the rotation and vibration of a well-bent triatomic molecule, such asH2 S, with a large, non-accessible barrier to linearity (of 24,423 ± 75 cm−1 [1]), a molecular kinetic energy operator (KEO) in terms of the 3 vibrational (e.g., two stretching and one bending) and three rotational coordinates (usually Euler angles) [2] is constructed.The three vibrational modes correspond to the standard bent-molecule methodology that is based on the 3N − 6 vibrational modes. The vibrational basis functions are effectively wavefunctions of a bent molecule and, they do not reflect the symmetry properties of a linear molecule, C∞v or
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