TURBOMOLE: Today and Tomorrow

TURBOMOLE is a highly optimized software suite for large-scale quantum-chemical and materials science simulations of molecules, clusters, extended systems, and periodic solids. TURBOMOLE uses Gaussian basis sets and has been designed with robust and fast quantum-chemical applications in mind, ranging from homogeneous and heterogeneous catalysis to inorganic and organic chemistry and various types of spectroscopy, light–matter interactions, and biochemistry. This Perspective briefly surveys TURBOMOLE’s functionality and highlights recent developments that have taken place between 2020 and 2023, comprising new electronic structure methods for molecules and solids, previously unavailable molecular properties, embedding, and molecular dynamics approaches. Select features under development are reviewed to illustrate the continuous growth of the program suite, including nuclear electronic orbital methods, Hartree–Fock-based adiabatic connection models, simplified time-dependent density functional theory, relativistic effects and magnetic properties, and multiscale modeling of optical properties

The Dalton quantum chemistry program system

Dalton is a powerful general\u2010purpose program system for the study of molecular electronic structure at the Hartree\u2013Fock, Kohn\u2013Sham, multiconfigurational self\u2010consistent\u2010field, M\uf8ller\u2013Plesset, configuration\u2010interaction, and coupled\u2010cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic\u2010structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge\u2010origin\u2010invariant manner. Frequency\u2010dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one\u2010, two\u2010, and three\u2010photon processes. Environmental effects may be included using various dielectric\u2010medium and quantum\u2010mechanics/molecular\u2010mechanics models. Large molecules may be studied using linear\u2010scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platform

Coupled-cluster techniques for computational chemistry: The CFOUR program package

An up-to-date overview of the CFOUR program system is given. After providing a brief outline of the evolution of the program since its inception in 1989, a comprehensive presentation is given of its well-known capabilities for high-level coupled-cluster theory and its application to molecular properties. Subsequent to this generally well-known background information, much of the remaining content focuses on lesser-known capabilities of CFOUR, most of which have become available to the public only recently or will become available in the near future. Each of these new features is illustrated by a representative example, with additional discussion targeted to educating users as to classes of applications that are now enabled by these capabilities. Finally, some speculation about future directions is given, and the mode of distribution and support for CFOUR are outlined

Density functional theory for large molecular systems

Nøyaktige simuleringer av kjemiske og biologiske prosesser på molekylært nivå har lenge vært uoppnålig for en rekke molekylære systemer, og har nå blitt mulig for mange av disse systemene gjennom ny metodeutvikling av Simen Reine, Trygve Helgaker og medarbeidere ved Universitetet i Oslo. Datasimuleringer er utbredt innen kjemi og relaterte felt som biologi, farmasi og medisin. Kvantekjemiske metoder er fundamentale for de mest nøyaktig simuleringsteknikkene, og er til stor hjelp ved bestemmelse og prediksjon av molekylære egenskaper, som for eksempel molekylers struktur, og gir i tillegg viktig og detaljert innsikt i kjemiske reaksjoner - både kvalitativt og kvantitativt. Anvendelsesområdet er nært knyttet til metodenes nøyaktighet, effektivitet og brukervennelighet. Utviklingen av nye og forbedrede metoder gjør oss i stand til å studere molekylære systemer som foreløpig har vært utenfor rekkevidde, og gir oss mer nøyaktig beskrivelse av de systemene vi allerede behandler idag. Som en konsekvens vil man kunne redusere bruken av kostbare og tidkrevende eksperimenter og samtidig hjelpe forskere verden over til bedre å forstå kjemiske mekanismer. De fleste kvantekjemiske beregninger som utføres idag benytter tetthetsfunksjonalteori (DFT), da denne metoden utgjør et godt kompromiss mellom nøyaktighet og beregningstid. Selv om DFT er meget nyttig, er dagens metoder begrenset til systemer bestående av noen få hundre atomer, og utelukker derfor en rekke systemer, for eksempel proteiner. I doktorgraden "Tetthetsfunksjonalteori for store molekylære systemer" har nye metoder innen DFT blitt utviklet med tanke på rutinemessige beregninger for store systemer. Beregninger for systemer med 1400 atomer er rapportert og metodene er i etterkant blitt benyttet for systemer med opp til 4000 atomer

Postmodern Electronic Structure Theory

This dissertation is concerned with the development and applications of approaches to the electron correlation problem. We start with an introduction that summarizes modern approaches to the electron correlation problem. In our view, there are two remaining challenges that modern density functional theory cannot satisfactorily solve. The first challenge is due to self-interaction error and the second is due to strong correlation. We discuss two methods developed by the author that attempt to make progress to address the second challenge.The first approach is useful in distinguishing strong and weak correlation in a computationally economical way. It is based on orbital optimization in the presence of regularized second-order Moller-Plesset perturbation theory (k-OOMP2), which is an approximate method to obtain Brueckner orbitals. k-OOMP2 includes weak correlation while attenuating strong correlation. As such, it distinguishes artificial and essential symmetry breaking which occur at the Hartree-Fock (HF) level. Artificial symmetry breaking appears due to the lack of weak correlation, not due to the lack of strong correlation. Therefore, the common wisdom in quantum chemistry, which equates symmetry breaking at the HF level and strong correlation, can result in a wrong understanding of the system. Essential symmetry breaking, on the other hand, signals strong correlation that is beyond the scope of simple perturbation theory. k-OOMP2 has been shown to reliably distinguish these two: symmetry breaking in the k-OOMP2 orbitals is essential. This has been applied to a recent controversy about whether C60 is strongly correlated. Starting from a broken-symmetry HF solution, k-OOMP2 restores every symmetry. As such, C60 is not strongly correlated. Moreover, k-OOMP2 successfully predicts strong correlation for a known biradicaloid, C36, by showing essential symmetry breaking in its orbitals. We also exploited essential symmetry breaking in singlet biradicaloids using k-OOMP2 and showed quantitative accuracy in obtaining singlet-triplet gaps of various molecules. This new approach should be helpful for redefining the common wisdom in quantum chemistry.The second method is an exact, spin-pure, polynomial-scaling way to describe strong spin-correlation (SSC). SSC is present when there are many spatially separate open-shell electrons with small spin-flip energy cost. Describing SSC exactly requires the inclusion of all essential spin-couplings. The number of such spin-couplings scales exponentially with the number of electrons. Because of this, SSC was thought to require an exponential number of wavefunction parameters in general. However, new development suggests that there is an efficient way to obtain all these spin-couplings with only a quadratic number of wavefunction parameters, which is called the coupled-cluster valence-bond (CCVB) method. We discuss different challenges in CCVB: (1) its non-black-box nature and (2) its inability to describe SSC in spin-frustrated systems. We present two improved CCVB approaches that address these two challenges. These approaches were applied to describe emergent strong correlation in oligoacenes and SSC in spin-frustrated systems such as single molecular magnets and metalloenzymes. The remaining challenges in CCVB are the inclusion of ionic excitations which are not relevant for SSC, but crucial for obtaining quantitative accuracy