The Molpro quantum chemistry package

Molpro is a general purpose quantum chemistry software package with a long development history. It was originally focused on accurate wavefunction calculations for small molecules but now has many additional distinctive capabilities that include, inter alia, local correlation approximations combined with explicit correlation, highly efficient implementations of single-reference correlation methods, robust and efficient multireference methods for large molecules, projection embedding, and anharmonic vibrational spectra. In addition to conventional input-file specification of calculations, Molpro calculations can now be specified and analyzed via a new graphical user interface and through a Python framework

Calculations of molecular vibrational spectra : methods and applications

Die Berechnung molekularer Schwingungsspektren mit quantenchemischen Methoden ist eine Routineanwendung. Aufgrund hoher Rechenzeiten wird in der Regel die harmonische Näherung zugrunde gelegt und die erhaltenen Frequenzen werden anschließend mit einem empirischen Faktor justiert. Eine Alternative besteht darin, nicht die Frequenzen, sondern die Kraftkonstanten zu skalieren. Damit kann man den unterschiedlichen Eigenschaften verschiedener Koordinatentypen Rechnung zu tragen, wodurch man genauere Ergebnisse erzielen kann. Im Rahmen diseser Arbeit wurden Beispiele aus der organischen Chemie gewählt, in denen berechnete Schwingungsspektren chemische Fragestellungen beantworteten. Das sind im Detail die Isomerentrennung polychlorierter Dibenzodioxine, die Strukturaufklärung des Azulens sowie die Reaktionsaufklärung in Umlagerungsreaktionen von Benzofuroxanen. Als Grundvoraussetzung dafür wurden übertragbare Faktoren für die Skalierung der Kraftkonstantenmatrizen bestimmt. Neben diesen Anwendungen werden neue ab-initio-Verfahren vorgestellt, die eine bessere Berücksichtigung von Elektronenkorrelationseffekten in Kraftkonstanten zum Ziel haben. Dabei wurden zwei Aspekte betrachtet: Zum einen wurde durch besondere Transformationstechniken und Näherungsverfahren versucht, bestehende Korrelationsmethoden effizienter zu machen, und zum anderen wurden prinzipiell neue Verfahren entwickelt. Alle Arbeiten zur Methodenentwicklung betreffen die lokalen Korrelationsmethoden von Pulay und Saebø. Für das lokale MP2-Verfahren wird eine neue Integraltransformation für die internen Austauschintegrale vorgestellt und die Berücksichtigung von Dispersionsenergien über ein neuronales Netzwerk. Im Rahmen der Neuentwicklung von ab-initio-Methoden wurden lokale QCISD-Energiegradienten entwickelt, deren Herleitung mit der konventionellen QCISD-Gradiententheorie verglichen wird. Es werden exemplarische Rechnungen an molekularen Strukturen sowie Schwingungsspektren vorgestellt.Calculating molecular vibrational spectra using quantum chemical methods is a standard application nowadays. Due to long computing times use is made of the harmonic approximation with a subsequent scaling of the obtained frequencies by empirically determined scaling factors. An alternative to this procedure is given by scaling the force constants rather than the vibrational frequencies. This opens the possibility to pay respect to the different nature of the internal coordinates. Consequently, significantly higher accuracy can be gained by this approach. In this work examples from the field of organic chemistry were chosen in which the computed spectra provide answers to specific problems. In detail, the identification of polychlorinated dibenzodioxines, the determination of the molecular structure of azulene, and the mechanistic examination of the molecular rearrangement of benzofuroxan were studied. As a prerequisite for these studies transferable scaling factors were determined for the scaling of the force constant matrices. Besides these applications novel ab initio techniques are presented which aim at better harmonic force constants by including electron correlation effects. Two aspects were considered in detail: (1) New transformation techniques and approximative treatments were investigated to enhance the efficiency of well-known methods. (2) Furthermore, new ab initio methods were developed. All studies considered here focus on the local correlation concept of Pulay and Saebø. For the local MP2 method a new integral transformation of the internal exchange matrices is presented. Dispersion energies are modeled by a neural network. With respect to the development of new ab initio methods local QCISD energy gradient theory was derived. A comparison of this theory with conventional QCISD gradient theory is provided. Benchmark calculations for molecular structures and vibrational spectra are presented

Anharmonic Franck–Condon Factors for the X̃²B₁ ← X̃¹A₁ Photoionization of Ketene

The X̃²B₁ ← X̃¹A₁ photoelectron spectra of ketene and its doubly deuterated isotopologue have been computed from correlated vibrational wave functions as determined from vibrational configuration interaction theory relying on multidimensional Born–Oppenheimer potential energy surfaces being obtained from explicitly correlated coupled-cluster calculations. Duschinsky effects were accounted for in all cases. Excellent agreement with available experimental data was achieved

Computational studies on 3-Aza-Cope rearrangements: protonation-induced switch of mechanism in the reaction of Vinylpropargylamine

The 3-aza-Cope rearrangements of 3-azoniahexa-1,5-diene (1), 3-azoniahex-1-ene-5-yne (3), and 3-azahex-1-ene-5-yne (5) were investigated up to the coupled-cluster level, CCSD(T), by using a valence triple-ζ basis set. Activation barriers and geometrical parameters of the transition states are provided. Conformational studies were performed for all reactants and products of the reactions. Solvent effects were estimated from self-consistent reaction field calculations. In contrast to the other two species, the Cope rearrangement of 5 was found to proceed by a stepwise mechanism

Spectroscopic Characterization of Diazophosphane—A Candidate for Astrophysical Observations

Quite recently, diazophosphane, HP−N≡N, was synthesized for the first time. This was accomplished by a reaction of PH _3 with N _2 under UV irradiation at 193 nm. As these two molecules have been observed in different astrophysical environments, as for example, in the circumstellar medium and, in particular, in the AGB star envelope IRC+10216, the question arises whether HPN _2 can be found as well. So far there is only the aforementioned experimental work, but neither rotational nor rovibrational data are available. Hence, the lack of accurate line lists, etc. to identify diazophosphane is the subject of this work, including a detailed analysis of the rotational, vibrational, and rovibrational properties for this molecule. Our calculations rely on multidimensional potential energy surfaces obtained from explicitly correlated coupled-cluster theory. The (ro)vibrational calculations are based on related configuration interaction theories avoiding the need for any model Hamiltonians. The rotational spectrum is studied between T = 10 and 300 K. In contrast, the partition functions for HPN _2 and DPN _2 are given and compared for temperatures up to 800 K. In addition, more than 70 vibrational transitions are calculated and analyzed with respect to resonances. All these vibrational states are considered within the subsequent rovibrational calculations. This allows for a detailed investigation of the infrared spectrum up to 2700 cm ^−1 including rovibrational couplings and hot bands. The results of this study serve as a reference and allow, for the first time, for the identification of diazophosphane, for example, in one of the astrophysical environments mentioned above