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Isotropic hyperfine coupling constants (iHFCCs) can be easily measured by electron spin resonance spectroscopy in solution, but they have proven difficult to calculate from first principles. We test the performance of the newly developed (aug-)cc-pVXZ-t5s basis sets for hydrogen with Dunning's (aug-)cc-pVXZ and -pCVXZ basis sets for non-hydrogen atoms. Correlation is included by CCSD and CCSD(T) using UHF and ROHF references. A two-point extrapolation of cc-pVDZ:cc-pVDZ-t5s-a5 and cc-pVTZ:cc-pVTZ-t5s-a6 hydrogen iHFCCs is found to be very useful. Diffuse functions have nearly no influence on extrapolated iHFCCs. We also explore the dependence of the calculated iHFCCs on the level of theory used in optimizing the geometries. For this purpose, we optimized geometries up to the UHF-CCSD/cc-pCVQZ and UHF-CCSDT/cc-pCVTZ levels and extrapolated to the "complete basis set" limit. The calculated iHFCCs are compared to reference values, which are experimental numbers corrected for solvent and the most important vibrational effects. Our test molecules are the CH 3 • , C 2 H 3 • , and H 2 CN • radicals. At the highest level of theory, the largest deviations from the reference values are smaller than 3.5 G and 6%. The rms errors are below 2.1 G and 4%. The cc-pVXZ:cc-pVXZ-t5s basis set combinations perform better than the EPR-n and the Chipman [631|41] basis set. All of them are better than similarly sized basis sets that were not developed for iHFCCs. The calculated iHFCCs are influenced most strongly by the choice of basis set, the perturbative inclusion of connected triple excitations, and the choice of reference wave function and the level of theory in geometry optimization. Core correlation is necessary for the computation of iHFCCs for non-hydrogen atoms but has very little influence on the iHFCCs of hydrogen atoms. A good compromise between the cost and accuracy of hydrogen iHFCCs seems to be reached by two-point extrapolated ROHF-CCSD(T)-fc iHFCCs at UHF-MBPT(2)-fc/cc-pVTZ geometries. ROHF-MBPT(2)-fc or UHF-CCSD-fc/cc-pVTZ geometries are necessary when single excitations are not negligible.

a b s t r a c tAccurate, experimental rotational-vibrational energy levels and line positions, with associated labels and uncertainties, are reported for the ground electronic state of the symmetric-top 14 NH 3 molecule. All levels and lines are based on critically reviewed and validated high-resolution experimental spectra taken from 56 literature sources. The transition data are in the 0.7-17 000 cm À 1 region, with a large gap between 7000 and 15 000 cm À 1 . The MARVEL (Measured Active Rotational-Vibrational Energy Levels) algorithm is used to determine the energy levels. Out of the 29 450 measured transitions 10 041 and 18 947 belong to ortho-and para-14 NH 3 , respectively. A careful analysis of the related experimental spectroscopic network (SN) allows 28 530 of the measured transitions to be validated, 18 178 of these are unique, while 462 transitions belong to floating components. Despite the large number of spectroscopic measurements published over the last 80 years, the transitions determine only 30 vibrational band origins of 14 NH 3 , 8 for ortho-and 22 for para-14 NH 3 . The highest J value, where J stands for the rotational quantum number, for which an energy level is validated is 31. The number of experimental-quality ortho-and para-14 NH 3 rovibrational energy levels is 1724 and 3237, respectively. The MARVEL energy levels are checked against ones in the BYTe firstprinciples database, determined previously. The lists of validated lines and levels for 14 NH 3 are deposited in the Supporting Information to this paper. Combination of the MARVEL energy levels with first-principles absorption intensities yields a huge number of experimental-quality rovibrational lines, which should prove to be useful for the understanding of future complex high-resolution spectroscopy on 14 NH 3 ; these lines are also deposited in the Supporting Information to this paper.

As a prototype of the SiOOOSi bonding region for silica modeling, especially the highly flexible SiOOOSi angle deformation, we studied the structure of disiloxane (H 3 SiOOOSiH 3 ), with ab initio calculations on the SCF, MBPT (2), CCSD, and CCSD(T) levels of theory. The convergence of the results is studied for a series of basis sets of increasing quality. Large basis sets including f functions are necessary to obtain reliable results for the structure and the barrier to linearization of the molecule. The following structure and energy parameters are the results of CCSD(T)-fc/cc-pVTZ calculations: SiOO distance is 1.645Å, the SiOOOSi angle 145.3°, and the barrier to linearization 0.48 kcal/mol.