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Periodic coupled cluster CCSD(T) for solids
Periodic coupled cluster CCSD(T) for solids extends the coupled cluster method with singles, doubles, and perturbative triples, molecular chemistry's "gold standard," to bulk crystals, molecular crystals, and adsorption on surfaces.
Periodic function
A periodic function, also called a periodic signal or periodic wave, repeats its values at regular intervals, like sine and cosine, which repeat every 2π radians.
Periodic law
The periodic law is the rule of chemistry that the properties of the elements recur periodically when ordered by atomic number, as the modern periodic table shows.
Periodic potential
A periodic potential is a potential energy function repeating at fixed spatial intervals, the standard quantum-mechanical model of an electron in a crystal, leading to energy bands and band gaps.
Periodic table
The periodic table is a tabular arrangement of the chemical elements in order of increasing atomic number, first published by Dmitri Mendeleev in 1869 to predict undiscovered elements.
Periodic trends
Periodic trends, also known as chemical periodicity, are the regular patterns in atomic size, ionization energy, electron affinity, and electronegativity across the rows and columns of the periodic table.
Periodically poled lithium niobate
Periodically poled lithium niobate (PPLN) is lithium niobate with periodically inverted ferroelectric domains, enabling efficient quasi-phase-matched frequency conversion via electric-field poling since the early 1990s.
Peripheral membrane protein
A peripheral membrane protein, or extrinsic protein, binds temporarily to the membrane surface through lipids or integral proteins and is released by high salt or extreme pH.
Periscope
A periscope is an optical instrument that lets an observer see over an obstacle through a tube with two 45-degree mirrors, used in submarines and trenches.
Peritectic point
A peritectic point is the invariant point on a phase diagram where, on cooling, a liquid and a previously formed solid react isothermally to produce a new solid phase.
Perkow reaction
The Perkow reaction is an organic reaction in which a trialkyl phosphite reacts with an α-halo carbonyl compound to give an enol phosphate, reported in 1952.
Perlite
Perlite is a hydrated volcanic glass that expands up to 20 times its volume when heated, forming a lightweight material used in construction, horticulture, and filtration.
Permafrost
Permafrost is ground, soil, or rock that stays at or below 0 °C for at least two years, underlying much of Alaska, Canada, Siberia, and Antarctica and storing large amounts of carbon.
Permafrost carbon feedback
The permafrost carbon feedback is warming amplified when thawing permafrost lets microbes decay ancient organic matter, releasing carbon dioxide and methane that warm the climate further.
Permanent (mathematics)
The permanent of a square matrix sums the product of entries over all permutations, like a determinant but with every term positive; computing it is #P-complete.
Permanent wilting point
The permanent wilting point (PWP) is the soil water content at which plants wilt irrecoverably, conventionally defined as water held at −1500 kPa, and with field capacity it bounds plant-available water.
Permanganate
Permanganate (MnO4−) is a tetrahedral manganese(VII) anion and strong oxidizer, known for its deep purple color and its use in water treatment, most familiar as potassium permanganate.
Permanganate oxidation
Permanganate oxidation is the oxidation of organic compounds and carbon materials by permanganate salts such as potassium permanganate, converting functional groups into diols, ketones, and carboxylic acids.
PERMANOVA
PERMANOVA (permutational multivariate analysis of variance) is a non-parametric method testing whether two or more groups differ in centroid location, introduced by Marti J. Anderson in 2001.
Permeability (electromagnetism)
Permeability, also known as magnetic permeability, is the physical quantity relating magnetic flux density B to field intensity H in a material, expressed in henries per meter.
Permeability (porous media)
Permeability, also called intrinsic permeability, is the capacity of a rock or porous material to transmit a fluid. It governs groundwater flow in aquifers and oil movement toward wells.
Permeameter
A permeameter is an apparatus that applies a known hydraulic gradient to water flowing through a soil sample, allowing its hydraulic conductivity to be determined.
Permeation barrier
A permeation barrier is a material or coating that blocks or retards the passage of gases and liquids, ranging from polymer composites to atomically thin graphene membranes.
Permian
The Permian is a geologic period from about 299 to 251 million years ago, the last of the Paleozoic Era, ending with the largest mass extinction on record.
Permian Basin (North America)
The Permian Basin is a large sedimentary basin underlying West Texas and southeastern New Mexico and one of the world's most prolific oil- and gas-producing regions.
Permittivity
Permittivity, also called dielectric permittivity, is the material property relating electric displacement to electric field, D = εE; in vacuum it equals ε₀, about 8.854 × 10⁻¹² F/m.
Permutation
A permutation is a rearrangement of the elements of a set, or a one-to-one mapping of a set onto itself; there are n! of n elements.
Permutation group
A permutation group is a group whose elements are permutations of a set, with composition as the operation; by Cayley's theorem, every group is isomorphic to one.
Permutation matrix
A permutation matrix is a square matrix with exactly one 1 in each row and each column, representing permutations of n objects; multiplying by it reorders rows or columns.
Permutation model
A permutation model, also called a Fraenkel–Mostowski model, is a model of Zermelo–Fraenkel set theory with atoms built by symmetric groups of permutations, devised to prove the Axiom of Choice independent.