Hugh Ross, in his famous book The Fingerprint of God (Whitaker House, 1989), summarized several important questions related to modern science:
(1) Is our universe finite or infinite in size and content?
(2) Has this universe been here forever or did it have a beginning?
(3) Was the universe created?
(4) If the universe was not created, how did it get here?
(5) If the universe was created, how was this creation accomplished, and what can we learn about the agent and events of creation?
(6) Who or what governs the laws and constants of physics?
(7) Are such laws the products of chance or have they been designed?
(8) How do the laws and constants of physics relate to the support and development of life?
(9) Is there any knowable existence beyond the apparently observed dimensions of the universe?
(10) Do we expect the universe to expand forever, or is a period of contraction to be followed by a big crunch?
While answers to some of these questions lie outside of the territory of chemistry, chemistry should help answering some of the last questions where molecules should play an important role. It is generally accepted that almost all present-day knowledge about the structure and properties of molecules comes via studies of their spectra. Although laboratory measurements are usually considered to be the prime sources for the relevant information, there are a number of occasions where theory has played, and will continue to play, a central role in the understanding of properties of molecules and their spectra. Cutting-edge studies in spectroscopy help to answer more mandane questions than those raised above but these questions and answers are still highly interesting to those studying nature.
Astronomical environments, such as those found in interstellar medium, are very different from those on Earth. This dissimilarity leads to a fundamentally different chemistry and to the production of species that can be hard to create in the laboratory. Theory can play an important role in predicting main features of spectra of such species or looking for possible spectral matches in other solar systems and in the atmospheres of exoplanets. The detailed understanding of the chemistry taking place in astronomical environments is central to understand a couple of less grand but still important questions:
(1) How did the solar systems form?
(2) Will the understanding of chemistry on earth help to understand chemistry in diverse astronomical objects?
(3) How did the building blocks of life form on earth and outside of earth?
(4) How did life begin on earth?
(5) Would understanding of the origin of life on earth help to understand the origin of life and its building blocks in other solar systems and on exoplanets?
Theoretical high-resolution molecular spectroscopy offers important contributions toward answering at least some parts of these questions.
Studies of potential energy and property hypersurfaces
Even when laboratory spectra have been recorded for a particular species, this data may only be partial. One such situation, which is particularly common for unstable or reactive species, is that wavelengths can be measured to high accuracy but there is no or extremely limited information on transition probabilities and line strengths. To understand these one needs to compute potential energy (PES) and property (like the dipole moment surface, DMS) hypersurfaces which can be obtained with modern techniques of electronic structure theory. Computing accurate and global PESs and DMSs is still a considerable challenge for polyatomic systems.
A. G. Császár, W. D. Allen, Y. Yamaguchi, and H. F. Schaefer III, Ab Initio Determination of Accurate Potential Energy Hypersurfaces for the Ground Electronic States of Molecules, in Computational Molecular Spectroscopy, 2000, eds. P. Jensen and P. R. Bunker, Wiley: New York.
A. G. Császár, G. Tarczay, M. L. Leininger, O. L. Polyansky, J. Tennyson, and W. D. Allen, Dream or Reality: Complete Basis Set Full Configuration Interaction Potential Energy Hypersurfaces, in Spectroscopy from Space, edited by J. Demaison, K. Sarka, and E. A. Cohen (Kluwer, Dordrecht, 2001), pp. 317-339.
Computational molecular spectroscopy
It must be realized that many modelling applications are particularly demanding on spectroscopic data. For example, to model the role of triatomic species, such as H2O or [H,C,N], which are important components of O-rich and C-rich cool stars, respectively, may require up to a billion vibration-rotation transitions. The laboratory measurement and analysis of a dataset of transitions of this size is completely impractical. Computational molecular spectroscopy, with its fourth-age quantum chemical techniques, come to the rescue and allows the straightforward determination of huge molecular linelists.
A. G. Császár and V. Szalay, Molekularezgések elméleti vizsgálata, in: Kémia Újabb Eredményei, Vol. 83, Akadémiai Kiadó: Budapest, 1998, pp. 213–353 (in Hungarian).
O. L. Polyansky, A. G. Császár, S. V. Shirin, N. F. Zobov, P. Barletta, J. Tennyson, D. W. Schwenke, and P. J. Knowles, High-Accuracy Ab Initio Rotation-Vibration Transitions of Water, Science 2003, 299, 539-542.
A. G. Császár, C. Fábri, T. Szidarovszky, E. Mátyus, T. Furtenbacher, and G. Czakó, Fourth Age of Quantum Chemistry: Molecules in Motion, Phys. Chem. Chem. Phys. 2012, 14(3), 1085-1106.