Quick search by citation:

42 U.S. Code § 7233 - Experts and consultants

The Secretary may obtain services as authorized by section 3109 of title 5, at rates not to exceed the daily rate prescribed for grade GS–18 of the General Schedule under section 5332 of title 5 for persons in Government service employed intermittently.

Statutory Notes and Related Subsidiaries
References in Other Laws to GS–16, 17, or 18 Pay Rates

References in laws to the rates of pay for GS–16, 17, or 18, or to maximum rates of pay under the General Schedule, to be considered references to rates payable under specified sections of Title 5, Government Organization and Employees, see section 529 [title I, § 101(c)(1)] of Pub. L. 101–509, set out in a note under section 5376 of Title 5.