First principles study of the OH radical: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi mathvariant="normal">B</mml:mi> <mml:mmultiscripts> <mml:mi mathvariant="normal">Σ</mml:mi> <mml:none/> <mml:mo>+</mml:mo> <mml:mpr

OH radical can be found in comets, stellar atmospheres, planetary atmospheres, interstellar clouds, exoplanets, the solar photosphere, and sunspots, making it a significant astrophysical molecule. Its detection relies on its emission properties, intrinsic to the molecules’ rovibrational levels. This work presents high-level calculations of the electronic structure of OH radical using complete active space self consistent field/multiReference configuration interaction calculations, including Davidson correction methods with large basis sets and extensive treatment of the electron correlation. The adiabatic potential energy curves of the low-lying doublet and quartet electronic states of the hydroxyl, in the representation <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msup> <a:mrow/> <a:mrow> <a:mn>2</a:mn> <a:mi mathvariant="normal">S</a:mi> <a:mo>+</a:mo> <a:mn>1</a:mn> </a:mrow> </a:msup> <a:msup> <a:mrow> <a:mi mathvariant="normal">Λ</a:mi> </a:mrow> <a:mrow> <a:mo>(</a:mo> <a:mrow> <a:mo>+</a:mo> <a:mo>/</a:mo> <a:mo>−</a:mo> </a:mrow> <a:mo>)</a:mo> </a:mrow> </a:msup> </a:mrow> </a:math> , are investigated. Additionally, a systematic investigation of the transition dipole moment function (TDMF) for the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mi mathvariant="normal">B</d:mi> <d:mmultiscripts> <d:mi mathvariant="normal">Σ</d:mi> <d:none/> <d:mo>+</d:mo> <d:mprescripts/> <d:none/> <d:mn>2</d:mn> </d:mmultiscripts> <d:mo>−</d:mo> <d:mi mathvariant="normal">X</d:mi> <d:mmultiscripts> <d:mi mathvariant="normal">Π</d:mi> <d:mprescripts/> <d:none/> <d:mn>2</d:mn> </d:mmultiscripts> </d:mrow> </d:math> and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:mi mathvariant="normal">D</i:mi> <i:mmultiscripts> <i:mi mathvariant="normal">Σ</i:mi> <i:none/> <i:mo>−</i:mo> <i:mprescripts/> <i:none/> <i:mn>2</i:mn> </i:mmultiscripts> <i:mo>−</i:mo> <i:mi mathvariant="normal">X</i:mi> <i:mmultiscripts> <i:mi mathvariant="normal">Π</i:mi> <i:mprescripts/> <i:none/> <i:mn>2</i:mn> </i:mmultiscripts> </i:mrow> </i:math> transitions has been done along with the Franck-Condon factors of these systems. The computed TDMF and potential energy function are used in Le Roy's LEVEL and Western's PGOPHER programs, in combination with experimental literature line positions, to calculate rovibronic <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"> <n:mrow> <n:mi mathvariant="normal">B</n:mi> <n:mmultiscripts> <n:mi mathvariant="normal">Σ</n:mi> <n:none/> <n:mo>+</n:mo> <n:mprescripts/> <n:none/> <n:mn>2</n:mn> </n:mmultiscripts> <n:mo>−</n:mo> <n:mi mathvariant="normal">X</n:mi> <n:mmultiscripts> <n:mi mathvariant="normal">Π</n:mi> <n:mprescripts/> <n:none/> <n:mn>2</n:mn> </n:mmultiscripts> </n:mrow> </n:math> and <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"> <s:mrow> <s:mi mathvariant="normal">D</s:mi> <s:mmultiscripts> <s:mi mathvariant="normal">Σ</s:mi> <s:none/> <s:mo>−</s:mo> <s:mprescripts/> <s:none/> <s:mn>2</s:mn> </s:mmultiscripts> <s:mo>−</s:mo> <s:mi mathvariant="normal">X</s:mi> <s:mmultiscripts> <s:mi mathvariant="normal">Π</s:mi> <s:mprescripts/> <s:none/> <s:mn>2</s:mn> </s:mmultiscripts> </s:mrow> </s:math> transitions. The corresponding “first principles”-based line lists include line intensities, line positions with the relevant quantum numbers of the upper and lower states, e/f parity, and Einstein <x:math xmlns:x="http://www.w3.org/1998/Math/MathML"> <x:mrow> <x:mi>A</x:mi> </x:mrow> </x:math> coefficients. The results show good agreement with previously published experimental and theoretical data, including a corresponding updated MARVEL (measured active rotational-vibrational energy levels) analysis of the lowest excited states of the OH molecule.