Abstract. Ionization potentials, bond dissociation ener- gies, and heat of formation for NH and NH + molecular species as well as for their elements were computed with highly reliable quadratic complete basis set and Gauss- ian-2 ab initio methods. The results are compared with experimental results and the assurance of these ab initio approaches is assessed. The same studies were also performed with three hybrid density functional methods (B3LYP, B3P86, and B3PW91) in combination with variously sized basis sets. The computational results are discussed in light of density functional theory reliability for exploring the potential energy of small polar molecular systems. Key words: Bond dissociation energies ± Ionization potentials ± Hybrid density functional theory 1 Introduction These days there is considerable interest in the compu- tational study of simple diatomic molecules that play a role in combustion and atmospheric chemistry. Two of them are NH and NH + . It is believed that these molecular species are involved in the combustion of nitroamine propelanes that are widely used as fuel in astronautic aeronautics [1, 2]. Furthermore, NH + is considered to be the ®rst step in the formation of ammonia in interstellar molecular clouds [3]. The most detailed information about these two molecular species is available from scattering experiments of the ion- molecule reaction N + +D 2 ® ND + + D [4]. This ex- perimental data was used by Tarroni and coworkers [5] to accurately evaluate the bond dissociation energies (BDEs) in NH and NH + molecular species [6]. Their results are supposed to be much closer to the expected values than results of previous computational studies obtained at MP4, CBSQ, and CASSCF ab initio levels of theory [7, 8]. These results have stimulated a series of ab initio calculations targeted to obtain highly accurate BDEs as well as the heat of formation (DH f,0 ) for NH and NH + [8±11]. In our opinion, there is sucient experimental data as well computational data, on these two molecular systems to allow performance of high-level ab initio and hybrid density functional theory (DFT) computational studies. Previously, we have demonstrated that some DFT methods are capable of accurately computing geometries [12±16], activation energies [17±19], BDEs [20±23], and ionization potentials (IPs) [24±26] for small polar molecules. It is therefore of interest to extend our computational studies to molecular systems of general interest to the atmospheric and combustion chemist. 2 Computational methods All computational studies were performed with the Gaussian 94 computational package [27]. For computation of very accurate energies Gaussian-2 (G2) [28] and the quadratic complete basis set (CBSQ) [29, 30] were used. There are three hybrid density func- tional methods that are a combination of Becke's three parameters exchange functional (B3) [31] with three dierent exchange func- tionals: (LYP) [32], (P86) [33], and PW91 [34]. These combinations make the B3LYP, B3P86, and B3PW91 hybrid DFT methods. In conjuction with these hybrid DFT methods 6-31+G(d), 6- 311+G(2d,2p),and 6-311+G(3df,3pd) Gaussian-type basis sets were employed [35]. These basis sets have been used by us to study many dierent chemical systems with DFT methods and have proven to produce reliable values. All experimental data used in this study were obtained from Ref. [5]. The DH f,0 for all molecules was computed directly from the total energies of elements from which they were built. For instance, the DHf,o of H + is its total energy, while the DH f,0 for the H atom is a half of the total energy dif- ference between two hydrogen atoms and one hydrogen molecule. Of course the DH f,0 for both H 2 and N 2 is by de®nition equal to zero. Regular article Computational studies of bond dissociation energies, ionization potentials, and heat of formation for NH and NH + . Are hybrid density functional theory methods as accurate as quadratic complete basis set and Gaussian-2 ab initio methods? Branko S. Jursic Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA Received: 21 July 1997 / Accepted: 8 December 1997 Theor Chem Acc (1998) 99:171±174 This article is supplemented by an internet archive which can be obtained electronically from the Springer-Verlag server located at http://link.springer.de/journals/tca