Information: - Theoretical chemistry seeks to provide explanations to the chemical and physical observations of molecules. Theoretical chemistry includes the fundamental laws of physics Coulomb's law, Kinetic energy, Potential energy, the virial theorem, Planck's Law, Pauli exclusion principle and many others to explain and predict chemical observed phenomena. - The hydrogen molecular ion, dihydrogen cation, or , is the simplest molecular ion. It is composed of two positively charged protons and one negatively charged electron, and can be formed from ionization of a neutral hydrogen molecule. It is of great historical and theoretical interest because, having only one electron, the Schrodinger equation for the system can be solved in a relatively straightforward way due to the lack of electronelectron repulsion (electron correlation). The analytical solutions for the energy eigenvalues are a "generalization" of the Lambert W function. Thus, the case of clamped nuclei can be completely done analytically using a computer algebra system within an experimental mathematics approach. Consequently, it is included as an example in most quantum chemistry textbooks. - A computer is a device that can be instructed to carry out an arbitrary set of arithmetic or logical operations automatically. The ability of computers to follow a sequence of operations, called a "program", make computers very flexible and useful. Such computers are used as control systems for a very wide variety of industrial and consumer devices. This includes simple special purpose devices like microwave ovens and remote controls, factory devices such as industrial robots and computer assisted design, but also in general purpose devices like personal computers and mobile devices such as smartphones. The Internet is run on computers and it connects millions of other computers. - In physics, classical mechanics is one of the two major sub-fields of mechanics, along with quantum mechanics. Classical mechanics is concerned with the set of physical laws describing the motion of bodies under the influence of a system of forces. The study of the motion of bodies is an ancient one, making classical mechanics one of the oldest and largest subjects in science, engineering and technology. It is also known as Newtonian mechanics, though textbook authors often consider Newtonian mechanics as one of the three main formalisms of classical mechanics, along with Lagrangian mechanics and Hamiltonian mechanics. - A computer program is a collection of instructions that performs a specific task when executed by a computer. A computer requires programs to function, and typically executes the program's instructions in a central processing unit. - Spartan is a molecular modeling and computational chemistry application from Wavefunction . It contains code for molecular mechanics , semi-empirical methods , ab initio models , density functional models , post-Hartree -- Fock models , and thermochemical recipes including G3 ( MP2 ) and T1 . Primary functions are to supply information about structures , relative stabilities and other properties of isolated molecules . Molecular mechanics calculations on complex molecules are common in the chemical community . Quantum chemical calculations , including Hartree -- Fock molecular orbital calculations , but especially calculations that include electron correlation , are more time consuming in comparison . Quantum chemical calculations are also called upon to furnish information about mechanisms and product distributions of chemical reactions , either directly by calculations on transition states , or based on the Hammond Postulate , by modeling the steric and electronic demands of the reactants . Quantitative calculations , leading directly to information about the geometries of transition states , and about reaction mechanisms in general , are increasingly common , while qualitative models are still needed for systems that are too large to be subjected to more rigorous treatments . Quantum chemical calculations can supply information to complement existing experimental data or replace it altogether , for example , atomic charges for QSAR analyses , and intermolecular potentials for molecular mechanics and molecular dynamics calculations . Spartan applies computational chemistry methods ( theoretical models ) to a number of a standard tasks that provide calculated data applicable to the determination of molecular shape ( conformation ) , structure ( equilibrium and transition state geometry ) , NMR , IR , Raman , and UV / visible spectra , molecular ( and atomic ) properties , reactivity and selectivity . - Molecular mechanics uses classical mechanics to model molecular systems. The potential energy of all systems in molecular mechanics is calculated using force fields. Molecular mechanics can be used to study molecule systems ranging in size and complexity from small to large biological systems or material assemblies with many thousands to millions of atoms. - Chemistry is a branch of physical science that studies the composition, structure, properties and change of matter. Chemistry includes topics such as the properties of individual atoms, how atoms form chemical bonds to create chemical compounds, the interactions of substances through intermolecular forces that give matter its general properties, and the interactions between substances through chemical reactions to form different substances. - Computational chemistry is a branch of chemistry that uses computer simulation to assist in solving chemical problems. It uses methods of theoretical chemistry, incorporated into efficient computer programs, to calculate the structures and properties of molecules and solids. It is necessary because, apart from relatively recent results concerning the hydrogen molecular ion (dihydrogen cation, see references therein for more details), the quantum many-body problem cannot be solved analytically, much less in closed form. While computational results normally complement the information obtained by chemical experiments, it can in some cases predict hitherto unobserved chemical phenomena. It is widely used in the design of new drugs and materials. - Computer simulations reproduce the behavior of a system using a model. Computer simulations have become a useful part of mathematical modeling of many natural systems in physics (computational physics), astrophysics, climatology, chemistry and biology, human systems in economics, psychology, social science, and engineering. Simulation of a system is represented as the running of the system's model. It can be used to explore and gain new insights into new technology and to estimate the performance of systems too complex for analytical solutions. - An experiment is a procedure carried out to support, refute, or validate a hypothesis. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when a particular factor is manipulated. Experiments vary greatly in goal and scale, but always rely on repeatable procedure and logical analysis of the results. There also exists natural experimental studies. - The many-body problem is a general name for a vast category of physical problems pertaining to the properties of microscopic systems made of a large number of interacting particles. "Microscopic" here implies that quantum mechanics has to be used to provide an accurate description of the system. A "large number" can be anywhere from 3 to infinity (in the case of a practically infinite, homogeneous or periodic system, such as a crystal), although three- and four-body systems can be treated by specific means (respectively the Faddeev and Faddeev-Yakubovsky equations) and are thus sometimes separately classified as few-body systems. In such a quantum system, the repeated "interactions" between particles create quantum correlations, or entanglement. As a consequence, the wave function of the system is a complicated object holding a large amount of information, which usually makes exact or analytical calculations impractical or even impossible. Thus, many-body theoretical physics most often relies on a set of approximations specific to the problem at hand, and ranks among the most computationally intensive fields of science. - A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge. However, in quantum physics, organic chemistry, and biochemistry, the term "molecule" is often used less strictly, also being applied to polyatomic ions. After reading the paragraphs above, we are interested in knowing the entity with which 'spartan ' exhibits the relationship of 'instance of'. Find the answer from the choices below. 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