• DFT
• Equilibrium Structure
• Geometry Database
• Rotational Spectra

The work presented in the following has been carried on in the SMART Labo-
ratory of the Scuola Normale Superiore, and is focused on computational aspects
of rotational spectroscopy. This thesis has benefited from the invaluable support
and collaboration of many people. Among them it is fair to mention professors
Vincenzo Barone, Cristina Puzzarini, and Malgorzata Biczysko, researchers Julien
Bloino and Nicola Tasinato, post-docs Matteo Piccardo and Daniele Licari, and the
student Marco Mendolicchio.
A first part of the work concerns the semi-experimental (SE) approach for de-
riving accurate equilibrium structures of medium-sized to large molecular systems
of organic and biological interest. SE equilibrium structures are derived by a least-
squares fit of the structural parameters to the experimental ground-state rotational
constants of several isotopic species corrected by vibrational contributions computed
by quantum mechanical (QM) methods. The B2PLYP double hybrid functional,
coupled with the correlation-consistent triple-ζ cc-pVTZ (VTZ) basis set, has been
validated in the framework of the semi-experimental approach for deriving accurate
equilibrium structures of molecules containing up to 15 atoms. A systematic com-
parison between new B2PLYP/VTZ results and several equilibrium SE structures
previously determined at other levels, in particular B3LYP/SNSD and CCSD(T)
with various basis sets, has put in evidence the accuracy and the remarkable stability
of such model chemistry for both equilibrium structures and vibrational corrections.
New SE equilibrium structures for phenylacetylene, pyruvic acid, peroxyformic acid,
and phenyl radical are discussed and compared with literature data.
In addition, the semi-experimental and theoretical equilibrium geometries of 10
sulfur-containing organic molecules, as well as 4 oxygenated ones, have been deter-
mined aiming both to provide new accurate data on some rather important chemical
moieties, only marginally represented in the literature of the field, and to examine
the structural features of carbon-sulfur bonds in the light of the so-called linear
regression approach. The structural changes issuing from substitution of oxygen
by sulfur are discussed to get deeper insights on how modifications in electronic
structure and nuclear potential can affect equilibrium geometries. With respect
to the previous set of molecules, we perform non-linear constrained optimizations
of equilibrium SE structures with a new general and user-friendly software under
development in our group, updating the definitions of useful statistical indicators.
This thesis contains more than 60 anharmonic force fields calculations together with
a full benchmark of B2PLYP equilibrium as well as semi-experimental geometries,
and all the aforementioned results have further enhanced our online database of
accurate semi-experimental equilibrium molecular geometries, that nowadays consists of data for 68 molecules, to be used in parametrization and validation studies,
as well as input information in many kinds of analyses and computations. It is
worth noting that two different research groups started to use the above discussed
databases in order to benchmark the accuracy of well known, and even new, density
functionals.
Particular attention has been devoted to the discussion of systems for which lack
of sufficient experimental data prevents a complete SE determination. In order
to obtain an accurate equilibrium SE structure for these situations, the so-called
templating molecule approach is discussed and generalized with respect to a previous
work. Important applications are those involving biological interesting molecules,
such as pyruvic acid. In addition, for more general situations the linear regression
approach has been proposed and validated.
Moreover, in line with the philosophy inspiring the so-called virtual multifrequency
spectrometer (VMS), part of the work discussed in this thesis concerns contributions
in the realization of a user-friendly black box tool able to support researchers in the
prediction, visualization and interpretation of microwave experiments. In order to
do so, VMS developers have started to interface VMS with the widely used SPFIT
and SPCAT programs, and preliminary tests have been made to reach the aimed
flexibility, robustness and reliability. Concerning the latter feature, exploiting both
the new tools of VMS and the SE equilibrium structures databases, an evaluation
of DFT performances in the prediction of the major rotational observables has been
carried on taking oxirane as test-case.