Calcium carbides: can hexa-carbides grow unlimitedly? Theoretical perspectives and issues that oppose a definite answer.

Quasi-molecule theory and 'tiles' embedded on the parent molecules are used to investigate whether calcium hexa-carbides can attain linearity irrespective of their size. By assuming that some small carbon clusters may behave pseudo-atomic-like, the approach is generalized to explore 'pseudo-linearity' among the calcium hexa-carbides. It is tentatively conveyed that the atomic basis sets used for studying families of related molecules, rather than individual ones, should be maintained constant for the whole set such as to warrant that any differences in the observed trends are not driven by computational approximations or numerical inaccuracies. In the absence of experimental data, low-hierarchy (often subminimal) basis-set calculations are employed for probing the predictions by quasi-molecule theory for the parent molecules, while augmented correlation-consistent basis sets can efficiently be used for accurate correlated calculations in the tiles they embed. Besides hexa-carbides, other calcium carbides, planar and volumetric, are also studied.