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Physics and Astronomy Classification Scheme® (PACS^®)

The Physics and Astronomy Classification Scheme® (PACS^®) is prepared by the American Institute of Physics (AIP) in collaboration with certain other members of the International Council on Scientific and Technical Information (ICSTI). The most recent internationally agreed scheme was published by ICSTI in 1991. Revised editions of PACS are published biennially, or as necessary, by AIP.

Link to both the current scheme and prior editions below:

PACS 2008 (latest version)
PACS 2006
PACS 2003

Introduction:

The Physics and Astronomy Classification Scheme® (PACS®) is a hierarchical subject classification scheme designed to classify and categorize the literature of physics and astronomy. PACS provides an essential tool for classification and efficient retrieval of literature in physics and astronomy; as such, PACS is used by AIP and other international publishers of journals in physics, astronomy, and related fields.

What is PACS:

PACS contains ten broad subject categories subdivided into narrower categories. The hierarchy includes mainly four levels of depth, with the narrowest term giving the most detailed characterization. However, beginning with the 2006 edition, a fifth level hierarchy was introduced; subsequently, in this new edition, the fifth level hierarchy is continued in sections that have undergone revision and will also be a part of future editions. PACS also includes detailed appendices for acoustics and geophysics, a nanoscale science and technology supplement, and a topical alphabetical index with corresponding PACS codes.

Depending on the topic, the most detailed PACS code may be found at the third, fourth, or fifth hierarchical levels. At these three levels, each PACS code consists of six alphanumeric characters divided into three pairs. The examples, in the table below, illustrate the structure and format of PACS codes for all levels of the scheme, using PACS codes where the hierarchy terminates at the third, fourth, and fifth levels:

Note that the use of uppercase and lowercase letters as the fifth character for fourth- and fifth-level codes, respectively, is a means to easily distinguish the level of a given code; the use of italics for the fifth level serves a similar purpose. However, case and font are not needed to determine uniqueness, i.e., there are no redundant codes.

How to use PACS:
In order to classify an article, the main topics presented in that article must be identified. The most specific PACS codes that describe the content of an article are then selected using the alphabetical index to PACS. The first code is reserved for the main topic of the paper. Select as many codes as are necessary to classify the paper; three to four codes are generally sufficient. For errata or related items, an additional code must be selected from 99.10.-x "Errata and other corrections".

What is new in PACS 2008:
There are extensive revisions in the following sections included in PACS 2008; these sections have been expanded with many new fourth- and fifth-level codes:

The minor revisions include additions of PACS codes, modifications of the text of PACS codes, and some PACS code deletions. The 2008 PACS Special Edition contains a full listing of PACS 2008 with new, modified, and deleted codes highlighted; the Special Edition serves as a bridge between PACS 2006 and 2008.

Availability of printed PACS:
Complimentary printed copies of PACS may be obtained by contacting pacs@aip.org (Scientific Classification Department, American Institute of Physics, Suite 1NO1, 2 Huntington Quadrangle, Melville, NY 11747-4502, USA).

Community Feedback
AIP welcomes feedback from the scientific community. Any comments or suggestions you may have, both on the scheme and on the form of presentation, may be sent to pacs@aip.org

Acknowledgments
American Institute of Physics (AIP) gratefully acknowledges the assistance and cooperation of the AIP Subcommittee on Classification and Information Retrieval (SCIR), consisting of appointed members representing a broad spectrum of scientific disciplines, which has oversight responsibility for PACS development. In addition, invaluable advice was provided by the members of the PACS Working Groups, and Editors of Member and Affiliated Society journals, as well as by the many advisors from the American Physical Society (APS), and by members of the physics community at large. Particular thanks are due to two long-time contributors: Stanley Brown, Editorial Director (retired) of the APS Journals, for his leadership and tireless efforts in support of PACS; and Safia Hameed, currently AIP's Scientific Classification consultant, for providing more than three decades of expert guidance in PACS development.

Members of both AIP's SCIR and the Working Groups formed under their charge serve on a voluntary basis. We express sincere appreciation to these dedicated individuals. Listed below are members of the AIP SCIR, Working Groups, and PACS 2008 Project Team, along with individual advisors, whose efforts were invaluable in producing this new edition of the Physics and Astronomy Classification Scheme: