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/* This file contains only Doxygen documentation on geometry objects.
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There is no actual code here.
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\defgroup geom The Geometry Object
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\brief Notes on geometry objects
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The geometry object is used in NWChem to store and manipulate important
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information describing the molecular system to be modeled, not all of
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which is specifically connected with the geometry of the system. The
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geometry object serves four main purposes;
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- provides a definition of the coordinate system and positioning in space
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(including lattice vectors for periodic systems)
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- defines an association of names/tags with coordinates in space
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- specifies the external potential (nuclear multipole
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moments, external fields, effective core potentials, ...) that
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define the Hamiltonian for all electronic structure methods
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- stores most Hamiltonian related information (but
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not wavefunction related information).
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The tag associated with a geometric center serves a number of purposes in
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NWChem. It provides a convenient and unambiguous way to refer to
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- a specific chemical element (which provides default values for information
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such as nuclear charge, mass, number of electrons, ...)
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- the name of an `atomic' basis set
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The tag can also serve as a test for symmetry equivalence, since lower symmetry
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can be forced by specifying different tags for otherwise symmetry equivalent
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The data contained in the geometry object (or information that can be derived
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from data in the object) include the following;
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- A description of the coordinates of all types of centers (e.g.,
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- Charges (or optionally, ECPs, ...) associated with those centers
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- Tags (names) of centers
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- Masses associated with centers
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- Variables for optimization (e.g., via constrained cartesians
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or Z-matrix variables)
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- Symmetry information
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- Any other simple scalar/vector attribute associated
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specifically with a center
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Specific geometries are referenced through an integer handle.
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Multiple geometries can be defined such that any one of them
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may be accessible at any instant for a given problem. However,
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geometries can consume a large amount of memory, so it is usually
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advisable to keep the number of simultaneously `open' geometries to a minimum.
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Logical functions return .true. on sucess, .false. on failure. Below the
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various functions are described in more detail.